This  book  is  DUE  on  the  last  date  stamped  below. 


HANDBOOK  OF  ROCKS 

FOR  USE 

WITHOUT  THE  MICROSCOPE, 

BY 

JAMES   FURMAN   KEMP,  E.M.,  D.Sc., 

PROFESSOR  OF  GEOLOGY  IN  THE  SCHOOL  OF  MINES,  COLUMBIA  UNIVERSITY, 
NEW  YORK. 


WITH  A 

GLOSSARY  OF  THE  NAMES  OF  ROCKS 

AND  OF   OTHER 

LITHOLOGICAL  TERMS. 


FIFTH  EDITION,  REVISED. 


NEW  YORK: 

D.  VAN    NOSTRAND    COMPANY, 

EIGHT  WARREN  STREET 

1921 

33359 


COPYRIGHT  1896,  1900, 1904, 1908,  1911,  1918 
BY  J.  F.  KEMP. 


N  ERA"P"NTING  COMPANY 
LANCASTER,  PA. 


PREFACE. 


The  clear  presentation  of  the  subject  of  rocks  to  beginners  is 
not  an  especially  simple  undertaking.  The  series  of  objects  is 
extremely  diverse,  and  many  unrelated  processes  are  involved  in 
their  production.  In  order  not  to  confuse  and  bewilder  students, 
the  teacher  must  emphasize  the  intelligible  points  and  the  recog- 
nizable characters,  avoiding  alike  distinctions  that  have  their  chief 
foundations  in  past  misconceptions,  such  as  the  time  element  in 
the  classification  of  igneous  rocks,  and  that  require  microscopic 
study  to  substantiate  them.  In  the  following  pages  the  attempt 
has  been  made  to  avoid  these  difficulties,  and  only  to  mention  and 
emphasize  the  characters  which  a  beginner,  properly  equipped  with 
the  necessary  preliminary  training  in  mineralogy,  can  observe  and 
grasp. 

Some  years  of  annually  going  over  this  ground  have  convinced 
the  writer  that  for  this  purpose  we  are  not  likely  to  reach  a  more 
serviceable,  fundamental  classification  than  the  time-honored  one 
of  Igneous,  Aqueous  (or  Sedimentary)  and  Metamorphic  rocks. 
They  furnish  not  alone  convenient  central  groups,  but  also  pre- 
pare the  student  for  subsequent  geological  reading.  With  the 
Aqueous  have  been  placed  the  Eolian  as  a  similar,  although  very 
minor  division,  so  that  fire,  water  and  air,  the  ancient  elementary 
agents,  are  emphasized  in  their  work  upon  the  earth,  and  the  fun- 
damental classification  is  based,  as  it  should  be,  on  method  of 
origin.  The  only  illogical  step  involved  is  the  placing  of  the 
breccias  together  with  the  sediments,  but  breccias  are  so  subordi- 
nate and  go  so  conveniently  with  conglomerates,  that  it  has  been  done. 

The  igneous  rocks  are  the  ones  which  present  the  greatest  diffi- 
culties to  the  learner.  In  the  following  pages,  after  a  preliminary 
exposition  of  principles,  the  very  minor  group  of  the  volcanic 
glasses  is  first  taken  up,  because  it  is  the  simplest  and  because  it 
illustrates  cooling  from  fusion  most  forcibly.  Passing  then  through 
the  felsitic  and  porphyritic  to  the  granitoid  textures,  rocks  of  in- 


IV 


PREFACE. 


creasing  complexity  are  one  after  another  attacked.  Analyses 
have  been  freely  used  to  illustrate  the  chemical  differences  of  mag- 
mas, because  in  no  other  way  can  the  varieties  be  fundamentally 
described.  Within  fairly  narrow  limits  the  chemical  composition 
of  the  magma  establishes  the  mineralogy  of  the  rock. 

The  Aqueous  and  Eolian  rocks  are  not  difficult  to  understand. 
The  metamorphic  are  in  many  respects  the  most  obscure  of  all, 
but  it  is  hoped  that  enough  varieties  have  been  selected  and  em- 
phasized to  serve  for  field  use  and  for  the  reasonably  close  deter- 
mination of  the  great  majority  of  those  that  will  be  met  in  Nature. 

Many  names  will  be  encountered  in  geological  reading  that  are 
not  mentioned  in  the  book  proper.  To  explain  them  and  to  avoid 
confusing  the  main  text  with  unessential  matter,  they  have  been 
compiled  in  a  Glossary.  Practically  all  the  names  for  rocks  will 
be  found  there,  and  some  related,  geological  terms.  The  chief 
guide  in  its  preparation  has  been  the  index  of  Zirkel's  great  Lehr- 
buch  der  Petrographie ;  but  not  a  few  American  terms  are  intro- 
duced, which  are  not  in  it  nor  in  Loewinson-Lessing's  Petrograph- 
isches  Lexikon,  to  which  the  writer  is  also  greatly  indebted.  Other 
works,  English,  French  and  American,  have  likewise  been  at 
hand.  One  only  .needs  to  compile  a  glossary  to  appreciate  what 
numbers  of  unnecessary  and  ill-advised  names  for  rocks  burden 
this  unfortunate  branch  of  science,  and  to  convince  one  that  the  phi- 
lological petrographer  comes  near  to  being  the  enemy  of  his  kind. 

So  far  as  possible,  technical  words  of  classical  derivation  have 
been  avoided  in  the  main  work  in  favor  of  simple  English,  and  for 
the  rocks  described,  American  types  have  been  especially  sought 
with  which  to  illustrate  the  different  species,  because  they  are  more 
significant  and  accessible  to  readers  on  this  side  of  the  ocean.  The 
text,  except  the  glossary,  appeared  as  a  series  of  papers  in  the 
School  of  Mines  Quarterly  during  1895-96.  J.  F.  K. 

AUGUST,  1896. 


NOTE  TO  THE  SECOND  EDITION. 

In  the  preparation  of  the  second  edition,  but  little  change  has 
been  made  in  the  main  text.  The  Glossary  has,  however,  been 
rewritten  and  brought  up  to  date.  J.  F.  K. 

DECEMBER,  1899. 


PREFACE   TO   THE   THIRD   EDITION. 


Several  important  changes  have  been  introduced  in  the  present 
edition,  chiefly  in  connection  with  the  igneous  rocks.  The  com- 
position of  the  minerals  entering  into  them  has  been  more  fully 
stated,  graphic  formulas  being  employed  where  they  seemed  de- 
sirable. The  ingenious  star-shaped  diagrams  which  were  first 
used  by  W.  C.  Broegger  for  single  analyses  and  subsequently  em- 
ployed by  W.  H.  Hobbs  for  composite  groups,  have  been  adapted 
to  the  analyses  here  selected  and  have  been  given  under  each  im- 
portant division  of  the  igneous  rocks-.  They  present  characteristic 
pictures  of  chemical  composition  which  are  well  adapted  to  em- 
phasize this  important  feature  for  beginners.  Whenever,  in  an 
analysis,  ferrous  iron  has  not  been  separately  determined,  it  has 
been  necessary  to  assume  a  value  for  it  on  the  basis  of  related 
analyses,  but  experiments  with  varying  values  have  shown  that 
within  the  limits  set  by  the  total  iron  oxides,  the  variation  in  the 
general  shape  of  the  figure  is  scarcely  appreciable. 

In  the  description  of  textures  and  their  development  much  greater 
stress  is  laid  than  formerly  upon  the  geological  occurrence  of  the 
rock  masses.  The  several  forms,  dikes,  sheets,  laccoliths,  etc., 
have  therefore  been  illustrated  by  cuts  and  in  the  table  of  classifi- 
cation, p.  23,  they  have  been  introduced  in  a  separate  column. 
The  igneous  rocks  have  also  been  treated  in  a  slightly  different 
way.  Thus,  having  established  a  series  of  analyses  characteristic 
of  a  certain  group,  as  for  instance  the  rhyolites  and  granites,  this 
magma  is  followed  through  the  several  textures  from  the  products 
of  a  quick  chill  to  those  of  slow  and  deep-seated  cooling.  In 
nearly  all  cases  four  stages  are  emphasized  and  a  uniform  nomen- 
clature is  employed.  Thus  we  have  the  Rhyolites,  Rhyolite- 
porphyries,  Granite-porphyries,  Granite,  and  similarly  for  all  the 
others.  The  diabases  present  the  one  exception  to  this  uniform 
treatment.  In  developing  the  above  plan  an  old  and  widely 
employed  nomenclature  has  been  used,  which  experience  of  some 


vi  PREFACE  TO    THIRD  EDITION. 

years  in  the  class-room  and  laboratory  leads  the  writer  to  believe, 
has  distinct  advantages. 

The  matter  relating  to  the  sedimentary  and  metamorphic  rocks 
has  not  been  essentially  changed.  A  chapter  has  been  added  on 
the  recasting  of  analyses  of  igneous  rocks,  which  may  serve  as  an 
introduction  to  the  Quantitative  Classification  of  Cross,  Iddings, 
Pirsson  and  Washington,  the  latter  being  too  complicated  for  an 
elementary  book.  Finally  the  Glossary  has  been  brought  up  to 
date. 

The  writer  is  greatly  indebted  to  his  colleagues,  Dr.  Charles  P. 
Berkey  for  advice  and  assistance  in  editing  and  Professor  A.  W. 
Grabau  for  suggestions  regarding  the  sedimentary  rocks. 

J.  F.  K. 

APRIL,  1904. 


NOTE  TO  THE   FOURTH  EDITION. 


In  the  fourth  edition  the  pages  relating  to  the  recasting  of  rock- 
analyses  (155-158)  have  been  somewhat  amplified,  and  factors  for 
turning  molecular  proportions  into  percentages  are  introduced 
(pp.  166-167).  An  appendix  to  the  Glossary  brings  the  rock- 
names  up  to  1908. 

J.  F.  K. 
APRIL,  1908. 


NOTE  TO  THE  FIFTH  EDITION. 

In  the  fifth  edition,  tables  of  the  sedimentary  and  metamorphic 
rocks  have  been  added,  and  additional  matter  regarding  the  recal- 
culation of  the  chemical  analyses  of  rocks  has  been  introduced. 
A  series  of  selected  analyses  of  the  igneous  rocks  has  been  further 
recast  so  as  to  show  the  relative  volumes  of  the  light  and  dark- 
colored  minerals.  The  writer  is  greatly  indebted  to  Professor  A. 
F.  Rogers  of  Stanford  University,  for  suggestions  and  for  aid  in 
bringing  the  Glossary  up  to  1911. 

J.  F.  K. 
AUGUST,  1911. 


ABBREVIATIONS. 


A.  A.  A.  S.,  or  Proc.  Amer.  Assoc.  Adv.  Sci. — Proceedings  of  the 
American  Association  for  the  Advancement  of  Science. 

Amer.  Geol.,  or  A.  G.  — American  Geologist. 

Amer.  Jour,  of  Sci.,  or  A.  J.  S.  — American  Journal  of  Science,  some- 
times called  Silliman's  Journal. 

Bull.  Geol.  Soc.  Amer.  —  Bulletin  of  the  Geological  Society  of  America. 

Bull.  Mus.  Comp.  Zool.  —  Bulletin  of  the  Museum  of  Comparative 
Zoology,  Harvard  University,  Cambridge,  Mass. 

Jahrb.  d.  k.  k.  g.,  Reichs. — Jahrbuch  der  kaiserlichen,  koniglichen 
Geologischen  Reichsanstalt,  Vienna,  Austria. 

Jour,  of  Geol.  —  Journal  of  Geology,  published  at  the  University  of 
Chicago. 

Neues  Jahrb.,  or  N.  J.  —  Neues  Jahrbuch  fur  Mineralogie,  Geologic  und 
Palaeontologie,  Stuttgart,  Germany. 

Quar.  Jour.  Geol.  Soc.,  or  Q.  J.  G.  S. — Quarterly  Journal  of  the 
Geological  Society  of  London. 

Tsch.  Mitth. — Tschermak's  Mineralogische  und  Petrographische  Mit- 
theilungen,  Vienna,  Austria. 

U.  S.  Geol.  Surv.  —  United  States  Geological  Survey,  Washington, 
The  Publications  are  Bulletins,  Monographs,  Annual  Reports. 
Folios  and  Professional  Papers. 

Zeits.  d.  d.  g.  Ges.  —  Zeitschrift  der  deutschen  geologischen  Geseli- 
schaft,  Berlin,  Germany. 

Zeits.  f.  Krys.  —  Zestschrift  ftir  Krystallographie,  Munich,  Germany. 


TABLE  OF  CONTENTS. 


Preface iii 

Abbreviations vi 

CHAPTER  I.  —  Introduction.  Rock-forming  Minerals.  Prin- 
ciples of  Classification I 

CHAPTER  II.  —  General  Introduction  to  the  Igneous  Rocks. 

Classification 15 

CHAPTER  III.  —  The  Igneous  Rocks,  continued.  The 
Glasses.  The  Rocks  whose  chief  feldspar  is  orthoclase. 
The  Phonolites  and  Nephelite-Syenites 25 

CHAPTER  IV.  —  The  Igneous  Rocks,  continued.  The  Dacite- 

Quartz-Diorite  Series  and  the  Andesite-Diorite  Series  .  56 

CHAPTER  V.  —  The  Igneous  Rocks,  continued.  The  Basalt- 
Gabbro  Series.  The  Feldspar-free  Basalts.  The  Pyroxe- 
nites  and  Peridotites.  Ultra-basic  Igneous  Rocks  .  .  69 

CHAPTER  VI.  —  Remarks  in  Review  of  the  Igneous  Rocks    .     88 

CHAPTER  VII.  —  The  Aqueous  and  Eolian  Rocks.  Introduc- 
tion. The  Breccias  and  Mechanical  Sediments  not  Lime- 
stones  92 

CHAPTER  VIII.  —  Limestones.  Organic  Remains  not  Lime- 
stones. Rocks  Precipitated  from  Solution.  Determina- 
tion of  the  Aqueous  and  Eolian  Rocks 105 

CHAPTER  IX.  — The  Metamorphic  Rocks.  Introduction.  The 

Rocks  Produced  by  Contact  Metamorphism     .     .     .     .121 

CHAPTER  X. — The  Metamorphic  Rocks,  continued.  The  Rocks 
Produced  by  Regional  Metamorphism.  Introduction. 
The  Gneisses  and  Crystalline  Schists 131 

CHAPTER  XI.  —  The  Metamorphic  Rocks,  continued.  The 
Rocks  Produced  by  Regional  Metamorphism.  The 
Quartzites  and  Slates.  The  Crystalline  Limestones  and 
Dolomites.  The  Ophicalcites,  Serpentines  and  Soapstones  144 


x  TABLE  OF  CONTENTS. 

CHAPTER  XII.  —  The  Metamorphic  Rocks,  concluded.  The 
Rocks  Produced  by  Atmospheric  Weathering.  The  De- 
termination of  the  Metamorphic  Rocks 1 54 

CHAPTER  XIII. — The  Recalculation  of  the  Chemical  Analyses 

of  Rocks 161 

Glossary 180 

Index 267 


LIST   OF   ILLUSTRATIONS. 

FlGURRS. 

1.  Dike  of  Andesite,  Los  Cerrillos,  N.  M.     .          .         .         .  15 

2.  Surface-flow,  Leucite  Hills,  Wyo.     .         .          .         .         .  15 

3.  Volcanic  Neck,  Boar's  Tusk,  Wyo.            .          .         .         .  15 

4.  Cross-section  of  Laccolith,  S.  D.                .          .         .          .  15 

5.  Intruded  Sheets,  near  New  Madrid,  N.  M.          .         .         .  16 

6.  Laccolith,  Ragged  Top  Mountain,  S.  D.  .         .         .         .  16 

7.  Geological  diagram  of  Fig.  6.            .  16 
8  and  9.   Diagrams  Illustrating  the  Rhyolites.           .         .         .  29 

10  and  n.   Diagrams  Illustrating  the  Granites.          ...  34 

12  and  13.   Diagrams  Illustrating  the  Trachytes.        ...  39 

14  and  15.   Diagrams  Illustrating  the  Syenites.  45 

1 6  and  17.   Diagrams  Illustrating  the  Phonolites.       ...  47 

1 8.  Platy  phonolite,  Sugar  Loaf  Mountain,  S.  D.,  facing.  .         .  48 

19  and  20.   Diagrams  Illustrating  the  Nephelite-syenites.   .          .  52 

21.   Diagram  Illustrating  Leucite  Rocks.  .         .         .         .         .  53 

22  and  23.   Diagrams  Illustrating  the  Dacites.            .         .          .  57 

24  and  25.   Diagrams  Illustrating  the  Andesites.        .         .         .  61 

26  and  27.  Diagrams  Illustrating  the  Diorites.           ...  65 

28  and  29.  Diagrams  Illustrating  the  Basalts.   .         .         .          .  70 

30  and  31.   Diagrams  Illustrating  the  Limburgites.     ...  74 

32.  Diabasic  Texture.    ........  77 

33  and  34.   Diagrams  Illustrating  the  Gabbros.         ...  79 

35  and  36.  Diagrams  Illustrating  the  Pyroxenites      ...  82 

37  and  38.   Diagrams  Illustrating  the  Peridotites.      ...  82 

39  and  40.   Diagrams   Illustrating  the  Quantitative  Mineralogy 

of  the  Igneous  Rocks 88 

41.  The  Formation  of  Limestones  from  Coral  Reefs.         .         .  107 


xi 


A  HAND  BOOK  OF  ROCKS. 

FOR  USE  WITHOUT  THE  MICROSCOPE. 


CHAPTER  I. 

INTRODUCTION.     ROCK-FORMING  MINERALS.     PRINCIPLES 
OF  CLASSIFICATION. 

A  rock  may  be  best  defined  as  any  mineral  or  aggregate  of  min- 
erals that  forms  an  essential  part  of  the  earth.  The  word  mineral 
is  used  because  this  is  our  most  general  term  for  all  inanimate 
nature,  and  while  the  lifeless  remains  of  organisms  often  contribute 
in  no  small  degree  to  rocks,  no  rock  is  made  up  of  those  which  are 
still  alive.  In  instances  a  single  mineral  forms  a  rock,  but  among 
minerals  this  is  the  exception.  By  far  the  greater  number  are  in 
such  small  amount  that  they  cannot  properly  be  considered  rocks. 
Rock-salt,  ice,  calcite,  serpentine,  cemented  fragments  of  quartz, 
kaolin  and  a  few  others  are  in  sufficient  quantity,  but  the  vast 
majority  of  rocks  consist  of  two  or  more.  The  condition  that  a 
rock  should  form  an  essential  part  of  the  earth  is  introduced  to  bar 
out  those  minerals  or  aggregates,  which,  though  important  in  them- 
selves, are  none  the  less  insignificant  as  entering  into  the  mass  of 
the  globe.  Thus  the  sulphide  ores,  while  locally  often  in  con- 
siderable quantity,  when  broadly  viewed  are  practically  neglectable. 
Yet  this  is  somewhat  arbitrary  and  there  are  single  minerals  and 
aggregates  that  may  properly  give  rise  to  differences  of  opinion. 
The  following  pages  err,  if  at  all,  on  the  side  of  demanding  that 
the  amount  should  be  large.  A  rock  must  also  have  an  individual 
character,  sufficient  to  establish  its  identity  with  satisfactory  sharp- 
ness. The  species  cannot  be  marked  off  with  the  same  definition 
as  in  plants,  animals,  or  minerals,  and  there  is  here  again  reason- 
able opportunity  for  differences  of  opinion  as  to  the  limits  which 
should  be  set,  some  admitting  of  finer  distinctions  and  greater  multi- 


2  A  HAND  BOOK  OF  ROCKS. 

plicity  of  species  than  others ;  but,  after  all  has  been  said,  there 
should  be  a  well-marked  individuality  to  each  rock  species  which 
any  careful  and  qualified  observer  may  readily  recognize.  Too 
great  refinements  and  too  minute  subdivisions  ought  to  be  avoided. 
The  determining  conditions  of  species  will  be  taken  up  at  greater 
length,  when  the  preliminaries  of  classification  have  been  set  forth, 
but  it  must  be  appreciated  that  the  point  of  view  is  also  a  most 
important  factor.  Thus  if  one  is  studying  the  geology  of  a  dis- 
trict with  close  accuracy,  and  is  tracing  out  the  history  and 
development  of  its  rocks  with  microscopic  determinations  and 
descriptions  of  minerals  and  structures  which  may  be  minute,  finer 
distinctions  will  naturally  be  drawn  than  those  that  suggest  them- 
selves to  one  who  is  engaged  in  ordinary  field  work  or  in  mining 
or  engineering  enterprises.  It  is  for  the  latter  class  that  these 
pages  are  prepared  and  throughout  the  descriptions  and  classifica- 
tion here  given,  the  necessary  limitations  and  the  practical  needs 
of  such  observers  are  always  kept  in  mind.  Textural  and  min- 
eralogical  distinctions  are  alone  emphasized  where  easily  visible  on 
a  specimen,  although  never  made  contradictory  of  principles  of 
origin  and  classification  which  could  be  carried  to  greater  length  and 
subdivision. 

Rocks  embrace  matter  in  a  great  variety  of  structures  and  con- 
ditions. While  in  general  we  picture  them  to  ourselves  as  solid, 
yet  under  the  terms  of  our  definition,  we  have  no  logical  right  to 
bar  out  liquids  or  even  gases.  The  physical  condition  may  vary 
with  ordinary  temperatures.  Thus  we  cannot  reject  ice  as  an  ex- 
tremely abundant  and  important  rock,  and  yet  its  solid  condition 
results  from  water  with  a  moderate  loss  of  heat,  and  at  ordinary 
temperatures  the  same  molecules  may  be  in  a  liquid  or  gaseous 
state.  All  that  we  know  of  volcanoes  indicates  that  liquid,  molten 
magmas  exist  for  long  periods  deep  in  the  earth,  yet  they  are 
none  the  less  rocks  because  of  their  liquidity.  In  general,  how- 
ever, rocks  are  solid,  and  gases  or  liquids  (except  water)  de- 
serve no  further  attention.  In  texture  rocks  may  be  loose  and  in- 
coherent as  in  sand,  gravel,  volcanic  dust  and  the  like,  or  they 
may  be  extremely  dense,  hard  and  solid,  as  in  countless  familiar 
examples.  This  solidity  or  massiveness  has  its  limitations,  for  all 
observation  and  experience  show  that  what  are  apparently  solid 


INTRODUCTION.  3 

masses  are  really  broken  up  by  multitudes  of  cracks  into  pieces 
of  varying  size.  All  quarries  and  mines  have  these,  and  they  may 
aid  or  annoy  the  operators  according  to  the  purposes  of  excava- 
tion. They  will  again  be  referred  to  at  length.  Unless  too  deep 
within  the  earth,  rocks  are  also  in  all  cases  permeated  with  minute 
pores  and  spaces  that  admit  of  the  penetration  of  water  and  other 
liquids,  especially  if  under  pressure.  These  are  important  factors 
in  terrestrial  circulations. 

THE  CHEMICAL  ELEMENTS  IMPORTANT  IN  ROCKS. 

The  chemical  elements  really  important  in  rocks  are  compara- 
tively few,  and  are  those  which  are  most  widespread  in  nature. 
The  best  estimate  which  has  been  made  is  that  of  F.  W.  Clarke,  in 
Bulletin  616  of  the  U.  S.  Geological  Survey,  p.  34,  1916' 
Analyses  of  all  the  accessible  terrestrial  matter,  including  the  air 
and  the  ocean,  have  been  used.  The  average  composition  of  the 
igneous  rocks  has  first  been  determined;  next  that  of  the  sedi- 
mentary types.  Since  the  metamorphic  rocks  represent  either 
igneous  or  sedimentary  originals,  they  do  not  call  for  special 
treatment.  The  average  composition  of  the  ocean  and  that  of 
the  atmosphere  have  been  computed.  In  the  general  composition 
of  the  rocky  crust  the  igneous  rocks  are  assigned  95  per  cent.; 
shale  4  per  cent.;  sandstone  0.75  per  cent.;  limestone  0.25  per 
cent.  In  the  final  average  the  rocky  crust  is  credited  with  93 
per  cent. ;  and  the  waters  with  7  per  cent.,  both  in  round  numbers 
since  the  atmosphere  is  but  0.03  per  cent,  of  the  total. 


Silicon  

,   25.80 

0.95 

0.08 

Aluminum.  .  .  , 

•   7-30 
.  4.18 

Titanium  .  
Chlorine.  . 

,.0.43 

O.2O 

Manganese.  . 

.  .0.08 

Calcium  , 
Magnesium  .  .  , 
Sodium  .  .  , 

.  3.22 
.   2.08 
.   2.36 

Carbon  
Phosphorus  .  .  , 
Sulohur.  .  . 

..0.18 

,  .0.11 
.  .0.11 

Strontium.  .. 
All  others.  .  . 

.  .  O.O2 
.  .0.47 

Total . . .  100.00 

It  is  of  interest  to  remark  that  of  all  the  common  metalsr 
useful  in  the  arts,  only  aluminum,  iron  and  manganese  appear 
in  the  table. 

There  is  good  ground  for  believing  that  toward  the  center  of  the 
earth  the  metallic  elements  become  much  more  abundant,  and 
that  near  the  center  some  of  the  heaviest  known  are  in  excess,  but 
these  inferences,  however  well-based,  concern  materials  far  beyond 


4  A   HAND  BOOK  OF  ROCKS. 

actual  experience,  and  of  no  great  moment  in  this  connection.  As 
regards  rocks  we  have  to  deal  with  the  outer  portions  of  the  globe, 
to  which  we  are  accustomed  to  refer  as  the  crust.  This  term  is  not 
meant  to  indicate  anything  as  to  the  condition  of  the  interior,  but 
merely  the  exterior  as  contrasted  with  the  inner  parts. 

The  chemical  elements  above  cited  are  combined,  except  per- 
haps in  volcanic  glasses,  in  the  definite  compounds  which  form 
mineral  species.  These  compounds  change,  more  or  less,  in  the 
course  of  time,  under  the  action  of  various  natural  agents,  chief  of 
which  are  water,  carbonic  acid  and  oxygen,  but  at  any  particular 
stage,  however  complex  the  rock  may  be,  it  is  made  up  of  definite 
chemical  compounds,  though  we  may  not  be  able  to  recognize 
them  all.  The  most  important  compounds  are  not  numerous  and 
are  practically  limited  to  the  following  :  silicates,  oxides,  carbo- 
nates, sulphates,  chlorides,  and  of  far  inferior  moment  phosphates, 
sulphides,  and  one  native  element  graphite. 

As  a  broad  conception  in  speaking  of  these  compounds  it  is 
in  many  respects  advantageous  to  have  the  igneous  rocks  pri- 
marily before  our  minds,  because  as  stated  above  they  are  the 
sources  of  the  others.  In  taking  up  the  minerals  the  purpose 
here  is  to  emphasize  their  chemical  composition  and  relative  im- 
portance, not  to  describe  them  as  would  be  done  in  a  text-book 
on  mineralogy  so  as  to  enable  a  student  to  recognize  them,  for 
such  preliminary  knowledge  is  here  assumed.  Our  purpose  is 
to  make  prominent  the  chief  chemical  compounds  entering  into 
the  earth,  and  to  prepare  the  way  for  a  true  conception  of  the 
range  and  relations  of  its  constituent  rocks. 

THE  SILICATES. 

THE  SILICATES  are  grouped  as  follows  :  the  feldspars  and  feld- 
spathoids;  the  pyroxenes;  the  amphiboles ;  the  micas;  olivine. 
The  last  four  groups  are  often  collectively  called  the  ferro-magne- 
sian  silicates.  Zircon  and  titanite  conclude  the  list  of  those  im- 
portant in  igneous  rocks.  In  addition  there  are  a  number  of 
others  that  are  especially  characteristic  of  altered  or  metamor- 
phosed rocks,  viz :  epidote,  scapolite,  garnet,  tourmaline,  topaz, 
andalusite,  cyanite,  fibrolite  or  sillimanite,  and  staurolite.  Finally 
a  few  hydrated  silicates  complete  the  list. 


INTRODUCTION.  5 

THE  FELDSPARS  and  their  related  minerals  are  all  double  sili- 
cates of  alumina  and  an  alkali  or  an  alkaline  earth  or  both.  We 
speak  of  them  as  alkali-feldspar,  potash-feldspar,  soda-feldspar, 
lime-soda  feldspar,  etc.,  based  on  this  fact.  They  are  generally 
grouped  as  orthoclase,  representing  monoclinic  feldspar  with  its 
two  cleavages  at  right  angles  (hence  the  name),  and  as  plagioclase 
or  triclinic  feldspar,  with  oblique  cleavages,  and  one  striated 
cleavage  plane.  Orthoclase  and  albite  are  salts  of  H4Si3O8  in 
which  one  monad  element  potassium  or  sodium  and  one  triad, 
aluminum,  satisfy  the  acid  radicle.  Anorthite,  on  the  other  hand, 
is  a  salt  of  2(H4SiO4),  one  dyad,  calcium,  and  two  triads,  aluminum, 
making  eight  bonds  of  affinity  in  all  being  required  to  satisfy  the 
acid  radicles.  Orthoclase  is  chiefly  K2O,Al2O3,6SiO2,  which  ex- 
panded form  may  be  condensed  to  KAlSi3O8.  Na2O  replaces  more 
or  less  of  the  K2O,  without  affecting  the  crystal  system.  Suf- 
ficient amounts  of  soda  are  however  capable  of  changing  the 
system  to  triclinic  and  the  feldspar  is  called  anorthoclase.  Micro- 
cline  is  also  a  triclinic  variety  of  potash  feldspar,  with  a  cleavage 
angle  slightly  less  than  a  right  angle,  but  with  peculiar  and  char- 
acteristic optical  properties,  which  are  chiefly  of  moment  in  micro- 
scopic work.  The  clear,  unclouded  orthoclase  of  the  later  volcanic 
rocks  is  often  called  sanidine.  It  does  not  differ  essentially  from 
the  orthoclase  of  the  older  rocks,  and  the  distinction  based  on 
geological  age  is  obsolete,  but  as  the  terms  are  still  used  in  the 
literature  of  the  subject  it  is  well  to  understand  them. 

The  plagioclase  feldspars  embrace  a  practically  unbroken  series 
from  pure  soda-alumina  silicate  in  albite,  Na2O,Al2O3,6SiO2  or 
when  condensed  NaAlSi3O8,  to  pure  lime-alumina  silicate,  anor- 
thite,  CaO,Al2O3,2SiO2  or  CaAl2Si2O8.  Various  mixtures  of  these 
two  molecules  give  the  intermediate  species,  but  the  two  on  which 
special  stress  is  ordinarily  placed  are  oligoclase,  with  soda  in 
excess  and  hence  called  soda-lime  feldspar,  and  labradorite  with 
lime  in  excess  and  hence  called  lime-soda  feldspar.  If  we  rep- 
resent the  orthoclase  molecule,  KAlSi3O8  by  Or;  the  albite 
molecule,  NaAlSi3O8  by  Ab;  and  the  anorthite,  CaAl2Si2O8  by 
An  ;  all  the  intermediate  feldspars  can  be  algebraically  expressed. 
Thus  anorthoclase  lies  between  Ab2Orj,  and  Ab^O^  ;  albite  em- 
braces those  from  Ab  through  Ab8Anj ;  oligoclase,  Ab6Anj, 


A  HAND  BOOK  OF  ROCKS. 


through  AbjAn!  (the  intermediate  mixtures  Ab3An2  through 
Ab4An3  are  called  andesine);  labradorite  includes  Ab,Anj  through 
AbjAn2  ;  bytownite  AblAn3—AblAn6  ;  anorthite  AbxAn8  to  An. 
This  conception  of  feldspars  as  isomorphous  mixtures  of  molecules 
is  a  very  valuable  one  and  by  determining  specific  gravity,  optical 
properties  and  chemical  composition,  one  or  all,  the  different 
members  can  be  identified.  Practically,  however,  in  the  ordinary 
determination  of  rocks,  aside  from  microscopic  work,  we  are  forced 
by  the  difficulty  of  distinguishing  the  intermediate  varieties,  into 
the  general  use  of  orthoclase  and  plagioclase,  and  rely  on  the 
presence  or  absence  of  the  striations  peculiar  to  the  basal  cleavage 
of  the  latter  in  distinguishing  between  the  two,  but  of  course  ex- 
perience and  familiarity  with  the  general  characters  and  associa- 
tions of  minerals  in  rocks  often  enables  one  to  determine  very 
closely  the  minor  varieties.  We  would  naturally  look  for  ortho- 
clase, albite  and  oligoclase  in  acidic  rocks  or  those  high  in  silica, 
while  in  basic  rocks  we  would  expect  those  near  the  anorthite  end. 

All  the  feldspars  have  very  similar  crystal  forms  when  these  are 
developed,  as  they  occasionally  are  in  rocks.  When  they  are 
small  and  irregularly  bounded,  cleavage  faces  should  be  sought 
out  and  examined  with  a  pocket  lense.  It  is  interesting  to  note 
that  only  in  igneous  rocks  do  we  obtain  crystals  uniformly  de- 
veloped on  all  sides,  for  only  in  a  fused  magma  do  they  swim  and 
grow  without  a  fixed  support. 

The  word  feldspar  is  spelled  by  English  writers  "  felspar,"  but 
among  Americans  the  more  correct  form,  based  on  the  etymology, 
is  employed,  following  the  German  original  "  Feldspath." 

FELDSPATHOIDS.  With  the  feldspars  are  placed  two  other  im- 
portant and  closely  related  minerals,  nephelite  and  leucite,  to 
which  may  also  be  added  one  that  is  quite  rare,  melilite.  Nephe- 
lite is  an  hexagonal,  soda-alumina  silicate,  4Na2O,4Al2O3,9SiO2,  in 
which  some  of  the  Na2O  is  replaced  by  K2O  and  CaO.  It  appears 
in  a  subordinate  series  of  igneous  rocks  that  are  rich  in  soda.  Leucite 
is  an  isometric  potash  silicate,  K2O,Al2O3,4SiO2,  with  a  little  Na2O 
replacing  part  of  the  K2O.  It  is  a  salt  of  metasilicic  acid,  HjSiOj, 
like  the  pyroxenes  and  amphiboles,  but  because  of  the  triad  ele- 
ment, aluminum,  it  is  necessary  to  have  two  of  the  acid  radicles. 
Thus  dividing  the  expanded  formula  given  above,  by  2  and  con- 


INTRODUCTION.  7 

densing  we  have  KAlSi2O6.  It  appears  as  an  important  rock- 
making  mineral  in  the  igneous  rocks  of  ten  or  fifteen  localities  the 
world  over,  and  is  therefore  of  very  limited  distribution.  Melilite 
is  an  extremely  basic,  lime-alumina  silicate,  i2CaO,2A!2O3,9SiO2, 
and  appears  in  a  few  rare  basalts. 

Reference  may  also  be  made  to  sodalite,  noselite  and  hauynite 
which  are  occasionally  met,  but  which  are  chiefly  of  microscopic 
interest. 

The  feldspars,  together  with  the  feldspathoids  nephelite  and 
leucite,  are  the  most  important  of  the  rock-making  minerals  in 
their  relations  to  the  classification  of  rocks. 

In  order  to  have  a  standard  series  of  analysis  with  which  to 
compare  those  of  rocks  later  given,  the  following  table  is  inserted 
of  theoretical  feldspars  and  feldspathoids.  The  relative  amounts 
of  the  several  oxides  will  suggest  the  extent  to  which  the  mole- 
cules are  present  in  any  rock  whose  analysis  is  known  : 

LKUCITB  MELILITK 

KAlSt.O.  CaltAl4Si,O,. 
55.0  38.1 

23.5  14.5 

21.5 

47-4 
2.48  2.93 

Recalling  what  has  been  said  about  the  replacement  of  the  alka- 
lies by  one  another,  and  that  we  never  meet  any  of  these  minerals 
chemically  pure,  according  to  the  formulas  above  given,  and  making 
suitable  allowance  for  this  replacement,  we  may  still  appreciate  that 
orthoclase  and  albite,  being  high  in  silica,  favor  acidic  rocks,  and 
the  others  being  low  in  silica,  basic  ones ;  that  nepheline  implies  a 
magma  rich  in  alumina  and  soda,  leucite  one  rich  in  potash,  and 
melilite  one  low  in  silica  and  alumina,  but  high  in  lime. 

THE  PYROXENES  and  the  AMPHIBOLES  are  best  described  to- 
gether. Each  embraces  a  series  of  compounds  of  the  same  chemical 
composition,  differing  only  in  physical  and  optical  properties.  As 
the  table  shows,  they  vary  from  magnesia  silicate  through  a  series 
of  lime  and  lime-alumina  silicates,  with  an  iron  silicate  generally 
present.  They  are  all  primarily  salts  of  metasilicic  acid,  H2SiOs. 
The  monad  and  dyad  bases,  sodium,  calcium,  magnesium  and 


OR 

AB 

AN 

KAlSijO. 

NaAlSi.O. 

CaAUSi.O. 

Na.Al.Si,0,4 

SO, 

64.7 

68.6 

43-i 

45.0 

A1.0, 

I8.4 

19.6 

36.8 

34-3 

K,O 

16.9 

Na,0 

ii.  8 

20.7 

CaO 

20.1 

Sp.  Gr. 

2.57 

2.62 

2.75 

2.58 

8  A  HAND  BOOK  OF  ROCKS. 

ferrous  iron  make  simple  and  easily  understood  compounds,  such 
as  Na20,  SiO2;  CaO,  SiO2 ;  MgO,  SiO2;  and  FeO,  SiO2.  On 
analysis  however  the  triad  bases,  ferric  iron  and  aluminum  are 
quite  invariably  found  in  augite  and  hornblende.  This  was  not 
easy  to  understand  until  the  following  replacement  was  suggested. 
If  we  write  the  formula  of  the  diopside  molecule  graphically  it  will 
appear  as  follows  :  Mg-O-Si=O 

A     i 

Ca— O— Si=0 

Now  in  the  lower  line  the  calcium  and  silicon  have  together  six 
bonds  of  affinity.  So  have  two  ferric  irons,  or  two  aluminums.  If, 
therefore,  we  replace  the  calcium  and  silicon  with  the  two  irons  we 

Willhave.  Mg-0-Si=0 

O  i 

I  /o\  I 
F<0> 


!/ 


This  is  a  common  molecule  in  augite.  Condensed  it  will  be  MgFe2- 
SiO6.  We  also  have  MgAl2SiO6.  The  explanation  will  serve  to 
make  clear  the  condensed  formulas  of  the  several  molecules  as 
given  in  the  table  below,  for  otherwise  the  molecules  with  the  triad 
elements  and  with  only  one  silicon  often  prove  very  puzzling.  The 
graphic  formula  for  diopside  will  also  show  how  two  bases  may 
enter  a  single  molecule.  All  the  pyroxenes  have  a  prismatic 
cleavage  of  nearly  90°  (87°  10'  or  thereabouts),  while  the  amphi- 
boles  cleave  along  a  prism  of  nearly  120°  (124°  n'). 

COMPOSITION.  PYROXENE.  AMPHIBOLB.  SYSTEM. 

(  MeOS'O  1  Enstatite  "| 

|  FeoSiO  *  [  Bronzite  Anthophyllite   \  Orthorhombic 

Hypersthene  J 

CaMgSi,O,  Diopside  Tremolite 

CaMgSi,06  i  Malacolite  Actinolite 

CaFeSijO,    /  (Diallage) 
CaMgSi,0,  I 
CaFeSLO. 

MgAl,SiO.  Augite  Hornblende         M°BOdllUC 

MgFe,SiO, 
FeAl,SiO.   J 

NaFeSi,0,  Acmite  Arfvedsonite 

jEgirine 


INTRODUCTION.  g 

Under  the  orthorhombic  pyroxenes  enstatite  has  least  of  the  mole- 
cule FeO,SiO2,  i.  e.,  FeO  less  than  5  per  cent. ;  bronzite  has  FeO 
more  than  5  and  less  than  14  per  cent ;  while  hypersthene  has  still 
higher  percentages  of  FeO.  The  increase  brings  about  a  darker 
color  and  changed  optical  properties.  The  orthorhombic  pyroxenes 
are  much  less  frequent  than  the  monoclinic,  but  are  of  wide  distri- 
bution, especially  hypersthene.  The  orthorhombic  amphiboles  are 
of  minor  importance  and  are  but  seldom  met. 

The  light-colored  monoclinic  pyroxenes  are  almost  pure  lime- 
magnesia  silicates,  and  are  called  diopside.  They  are  chiefly  found 
in  crystalline  limestones.  As  iron  increases,  they  pass  into  malaco- 
lite,  which  may  also  contain  small  amounts  of  the  aluminous  mole- 
cules. Neither  of  these  pyroxenes  is  of  special  abundance  as  a 
rock-maker.  When  pinacoidal  cleavages  around  the  vertical  axis 
appear  in  addition  to  the  prismatic  ones,  in  pyroxenes  of  the  general 
composition  of  malacolite  they  are  called  diallage  and  are  important 
in  some  igneous  rocks.  But  the  chief  rock-making  pyroxenes  are 
the  dark  aluminous,  ferruginous  ones,  which  are  called  augite,  and 
these  are  among  the  most  important  of  all  minerals  in  this  con- 
nection. The  igneous  rocks  rich  in  soda,  in  which  nepheline  is 
common,  are  the  ones  that  contain  acmite  and  aegirite,  the  soda- 
pyroxenes. 

The  monoclinic  amphiboles  are  closely  parallel  in  their  occur- 
rence and  relations  to  the  pyroxenes.  Tremolite  is  met  in  crystal- 
line limestones.  Actinolite  may  form  schistose  rocks  by  itself,  but 
much  the  most  important  variety  is  hornblende,  the  aluminous 
variety  corresponding  to  augite.  The  soda  amphibole,  arfvedson- 
ite,  is  rare. 

The  pyroxenes  and  amphiboles  are  often  collectively  re- 
ferred to  as  the  bisilicates,  the  oxygen  of  the  base  being  to  the 
oxygen  of  the  silicon,  as  shown  in  the  first  two  formulas,  in 
the  ratio  of  1:2.  It  is  also  interesting  to  note  that  many  blast 
furnace  slags  are  calculated  on  the  basis  of  the  formulas  for 
pyroxene. 

THE  MICAS.  All  the  micas  are  salts  of  orthosilicic  acid,  H4SiO4. 
This  acid  radicle  will  be  satisfied  by  one  monad  base  and  one  triad 
together,  such  as  potassium  and  aluminum,  but  as  dyad  bases  also 
occur  with  both  these  it  is  necessary  to  assume  multiples  of  the 


10 


A  HAND  BOOK  OF  ROCKS, 


orthosilicic  acid.     Thus  muscovite,  the  simplest  of  the  common 
micas,  has  a  graphic  formula  like  this 


K— O— 
— O— 


For  biotite  which  is  more  complex  we  need  three  molecules  of 
the  acid  as  follows : 


Mg 


— O— 


Al- 


Fe~0- 
H— O— 

Ferric  iron  may  take  the  place  of  the  aluminum,  and  various 
other  variations  may  be  made  of  this  general  formula.  When  con- 
densed, the  formula  for  muscovite  will  be  HKAl2Si2O8,  and  the 
variety  of  biotite  given  above  will  be  HKMgFeAl2Si3O12. 

Biotite  is  the  dark  mica  and  is  much  the  commonest  of  the 
group.  It  is  very  widespread,  and  is  easily  recognized  by  its 
cleavage  —  even  small  crystals  can  be  picked  apart  into  leaves  with 
the  point  of  a  knife.  Biotite  is  often  called  magnesia-mica.  It 
enters  into  the  classification  of  igneous  rocks  in  an  important  way. 
Phlogopite  is  a  lighter-colored  but  closely  related  variety,  which 
favors  crystalline  limestones.  Muscovite,  from  its  richness  in  pot- 
ash, is  often  called  potash-mica.  It  is  widespread  in  granites  and 
schists.  In  composition  it  closely  resembles  orthoclase. 

OLIVINE,  the  unisilicate  of  magnesium  and  iron,  2(Mg,  Fe)O,  SiO2, 
completes  the  list  of  silicates  which  are  of  the  first  order  of  im- 
portance in  igneous  rocks.  The  above  name  is  usually  employed 
in  preference  to  chrysolite.  Olivine  is  practically  limited  to  basic 
igneous  rocks.  Like  the  micas  it  is  an  orthosilicate. 

Zircon  and  titanite  are  interesting  microscopic  accessories,  but  as 
rock-making  minerals  they  are  seldom  visible  to  the  naked  eye. 


INTRODUCTION.  n 

Along  the  contacts  of  intrusions  of  heated  igneous  rocks,  and  in 
regions  where  the  original  sediments  have  undergone  strong  dy- 
namic disturbances,  with  oftentimes  attendant  circulations  of  waters 
more  or  less  heated,  a  series  of  characteristic  silicates  is  in  each 
case  developed.  Garnet,  tourmaline,  topaz,  andalusite,  scapolite 
and  biotite  are  especially  characteristic  of  the  former;  garnet, 
cyanite,  silUmanite,  staurolite,  biotite,  and  muscovite  of  the  latter. 
Details  of  the  development  and  associations  of  each  of  these  groups 
are  subsequently  given  under  the  metamorphic  rocks.  Epidote 
results  when  feldspars  and  the  ferro-magnesian  silicates  undergo 
decay  and  alteration  in  proximity,  so  that  the  solutions  afforded 
may  react  on  one  another. 

The  hydrated  silicates  of  chief  importance  include  a  magnesian 
series,  embracing  talc  and  serpentine,  which  result  from  the  ferro- 
magnesian  minerals ;  a  ferruginous  aluminous  series,  with  much 
iron  oxide,  usually  collectively  called  "chlorite,"  and  derived  from 
the  iron-alumina  silicates  ;  and  finally  kaolin,  the  hydrated  silicate 
of  alumina  that  is  chiefly  yielded  by  feldspar.  Zeolitic  minerals 
are  also  often  met,  but  rather  as  vein  fillings  and  in  amygdaloidal 
cavities  than  as  important  rock  makers. 

The  oxides  include  quartz  and  its  related  minerals  chalcedony 
and  opal,  and  the  oxides  of  iron  —  magnetite  and  hematite  and  the 
hydrated  oxide,  limonite.  With  these  should  be  mentioned  chro- 
mite  and  ilmenite  (menaccanite),  which  are  of  minor  importance. 
Quartz  is  found  in  all  rocks  high  in  silica.  Magnetite  and  hema- 
tite are  at  times  almost  abundant  enough  to  constitute  rocks  them- 
selves. They  favor  igneous  and  metamorphic  varieties  when  pres- 
ent in  a  subordinate  capacity.  Magnetite  is  the  most  widespread 
of  all  the  rock-making  minerals.  Limonite  is  an  alteration  prod- 
uct. Chromite  is  practically  limited  to  the  basic  igneous  rocks  and 
their  serpentinous  derivatives.  Ilmenite  is  a  common  accessory  in 
many  igneous  rocks. 

The  carbonates  are  calcite,  dolomite  and  siderite,  all  three  being 
really  members  of  an  unbroken  series  from  pure  carbonate  of  cal- 
cium, through  admixtures  of  magnesium  carbonate  to  pure  magne- 
site  on  the  one  hand  or  with  increasing  carbonate  of  iron  to  pure 
siderite  on  the  other.  The  sulphates  of  moment  are  anhydrite  and 
gypsum,  the  latter  the  hydrous,  the  former  the  anhydrous  salt  of 


12  A  HAND  BOOK  OF  ROCKS. 

lime.  The  one  chloride  is  the  sodium  chloride,  rock  salt  or  halite, 
and  the  one  phosphate  is  apatite,  which  is  a  phosphate  and  chloride 
of  lime.  The  two  sulphides  of  iron,  pyrite  and  pyrrhotite  are 
the  only  ones  sufficiently  widespread  to  deserve  mention,  and 
graphite  is  the  chief  representative  of  the  elementary  substances, 
although  native  sulphur  might  perhaps  with  propriety  be  also  men- 
tioned. 

We  speak  of  minerals  as  essential  and  accessory,  meaning  by 
the  former  term  those  that  constitute  a  large  part  of  the  rock,  and 
that  must  be  mentioned  in  the  definition  ;  by  the  latter  those  that 
are  present  in  small  amounts  or  that  are  more  or  less  fortuitous. 
Primary  minerals  are  those  that  date  back  to  the  origin  of  the 
rock,  as  for  instance  the  ones  that  crystallize  out  from  a  molten 
magma  as  it  solidifies ;  secondary  minerals  are  formed  by  the 
alteration  of  the  primary.  Feldspars,  pyroxene  and  hornblende 
are  good  illustrations  of  the  former;  hydrated  silicates  of  the 
latter. 

THE  PRINCIPLES  UNDERLYING  THE  CLASSIFICATION  OF  ROCKS. 

Rocks  must  of  necessity  be  classified  in  order  to  place  them  in 
their  natural  relations  so  far  as  possible  and  to  allow  of  their  syste- 
matic study.  At  the  same  time  they  are  so  diverse  in  their  nature 
and  origin  that  the  subject  is  not  an  easy  one.  They  must  however 
be  grouped  on  the  basis  of  their  structures  and  textures  ;  or  of  their 
mineralogical  composition  ;  or  of  their  chemical  composition  ;  or  of 
their  geological  age  ;  or  of  their  method  of  genesis.  One  or  several 
of  these  principles  enter  into  all  schemes.  On  the  basis  of  the 
first,  rocks  have  been  classified  as  massive  and  stratified  ;  as  crystal- 
line and  fragmental  or  clastic,  each  with  subdivisions  on  one  or 
more  of  the  other  principles.  On  the  basis  of  the  second  we  have 
had  those  with  only  one  mineral  (simple  rocks)  and  those  with 
several  (complex  rocks).  The  chemical  composition  as  shown  by 
a  total  analysis  (bausch-analysis),  without  regard  to  special  mineral 
components,  is  of  almost  universal  application  in  a  subordinate 
capacity.  It  must  be  regarded  in  the  group  of  igneous  rocks  and 
in  those  that  are  deposited  from  solution,  chiefly  highly  calcareous 
or  highly  siliceous  rocks.  The  principle  of  geological  age  was 
formerly  much  valued  in  connection  with  the  igneous  rocks,  but  it 


INTRODUCTION.  13 

is  a  thoroughly  exploded  one.  The  principle  of  origin  or  genesis 
is  the  most  philosophical  of  all  as  a  fundamental  basis,  but  while 
in  the  greater  number  of  cases  it  may  be  readily  applied  there  are 
some  puzzling  members  whose  entire  geological  history  is  not  well 
understood.  Very  early  in  the  development  of  the  subject  it  was 
appreciated  that  there  were  two  great,  sharply  contrasted  groups, 
according  as  the  rocks  had  consolidated  and  crystallized  from  a 
molten  condition  or  had  been  deposited  in  water  either  as  mechan- 
ical fragments  or  as  chemical  precipitates.  Widened  observation, 
especially  in  arid  and  sandy  regions,  has  added  to  these  a  less  im- 
portant group  of  those  whose  particles  have  been  heaped  together 
by  the  wind.  They  are  called  the  eolian  rocks  and  will  be  taken  up 
together  with  the  aqueous,  with  which  they  have  many  points  in 
common.  Two  grand  divisions  have  therefore  been  established, 
the  igneous,  on  the  one  hand,  and  the  aqueous  and  eolian  on  the 
other. 

Even  a  limited  field  experience  soon  convinces  the  observer  that 
many  rocks  are  encountered  which  cannot  be  readily  placed  with 
either  of  the  two  great  classes  whose  origin  is  comparatively 
simple.  Rocks  for  instance  are  met  having  the  minerals  common 
to  the  igneous  but  with  structures  that  resemble  those  of  sediments 
in  water. 

Great  geological  disturbances,  especially  if  of  the  nature  of  a 
shearing  stress,  may  so  crush  the  minerals  of  any  igneous  rock 
and  stretch  them  out  in  bands  and  layers  as  to  closely  imitate  a 
recrystallized  sediment.  The  baking  action  of  igneous  intrusions 
on  fine  sediments,  such  as  clays  and  muds,  makes  it  difficult  for 
an  observer,  without  the  aid  of  thin  sections  and  a  microscope,  to 
say  where  the  former  sediment  ends  and  the  chilled  magma  begins. 
Sediments  buried  at  great  depths  and  subjected  to  heat  and  hot 
water  become  recrystallized  with  their  chemical  elements  in  new 
combinations.  These  excessively  altered  rocks  have  been  often 
grouped  into  a  separate,  so-called  "  metamorphic  "  division,  which 
was  a  sort  of  "  omnibus  "  of  unsolved  geological  problems.  This 
metamorphic  group  is  useful,  and  the  term  is  a  common  one  in  the 
science,  but  wherever  possible  it  is  well  to  appreciate  the  true 
affinities  of  its  members  which  though  altered  are  still  referable  to 
their  originals. 


14  A  HAND  BOOK  OF  ROCKS. 

In  the  following  pages  these  three  divisions  will  be  adopted,  but 
the  metamorphic  group  will  be  reduced  to  a  minimum  by  remark- 
ing, in  connection  with  descriptions  of  the  unaltered  rocks,  the 
changes  that  igneous  and  aqueous  undergo. 

We  take  up,  therefore,  in  this  order  : 

A.  The  Igneous  Rocks. 

B.  The  Aqueous  and  Eolian  Rocks. 

C.  The  Metamorphic  Rocks. 


FIG.  I.  Dike  of  andesite  15  ft.  thick  and  50  ft.  high,  cutting  sandstones.  Ortiz 
arroyo,  near  Los  Cerrillos,  N.  M.  D.  W.  Johnson,  School  of  Mines  Quarterly,  July, 
1903,  461. 


W 


FIG.  2.     Surface  flow.     Black  Rock  Mesa,  Leucite  Hills,  Wyo.     Kemp  and  Knight,  Bulletin  Geol.  Soc. 

Amer.,  XIV.,  323,  1903. 


FIG.   3.     Volcanic  Neck.     The  Boar's  Tusk,    Leucite   Hills,    Wyo.     Kemp  and  Knight,  Bull.   Geol. 
Soc.  Amer.,  XIV.,  328,  1903. 


FIG.  4.     Laccolith.     Cross-section  of  the  Ragged  Top  laccolith  of  phonolite,  Black  Hills, 
S.  D.     J.  D.  Irving,  Annals  N.  Y.  Acad.  Sci.,  XII.,  Plate  VI.,  1899. 


CHAPTER  II. 
GENERAL  INTRODUCTION  TO  THE  IGNEOUS  ROCKS.     CLASSIFICATION. 

The  Igneous  rocks  are  first  treated  because  they  have  been  the 
originals,  according  to  our  best  light,  from  which  all  the  others 
have  been  directly  or  indirectly  derived,  for  either  from  the  frag- 
ments, as  afforded  by  their  decay,  or  from  the  mineral  solutions, 
yielded  by  their  alteration,  possibly  in  the  primitive  history  of  the 
globe,  all  the  others  have  been  produced. 

The  igneous  rocks  occur  in  dikes,  sheets,  laccoliths,  bosses  and 
vast  irregular  bodies,  for  which  we  have  no  single  term.  Dikes 
(spelled  also  dykes)  have  penetrated  fissures  in  other  rocks,  and 
have  solidified  in  them.  They  therefore  constitute  elongated  and 
relatively  narrow  bodies,  of  all  sizes,  from  a  fraction  of  an  inch  in 
thickness  and  a  few  feet  in  length,  to  others  a  thousand  or  more 
jeet  across  and  miles  in  length.  Sheets  are  bodies  of  relatively 
great  lateral  or  horizontal  extent,  compared  with  their  thickness. 
They  are  either  surface  flows,  which  may  be  afterwards  buried  or 
else  are  intruded  between  other  strata.  In  the  last  case,  if  len- 
ticular in  shape,  they  are  often  called  laccoliths.  Roughly  cylin- 
drical masses,  such  as  might  chill  in  the  conduit  of  a  volcano,  are 
called  necks.  Irregular,  projecting,  rounded  bodies  are  called 
bosses.  The  enormous  masses  of  crystalline  rocks  like  granite 
that  often  cover  hundreds  of  square  miles,  and  that  frequently 
appear  to  have  fused  their  way  upward  by  melting  overlying  rocks 
into  their  substance,  are  called  batholiths.  They  have  in  most 
if  not  all  instances,  only  been  uncovered  by  erosion,  for  the  name 
means  a  rock  belonging  to  the  depths  of  the  earth.  It  will  be 
later  brought  out  that  the  character  of  the  occurrence,  whether  as 
dike,  surface  flow,  intruded  sheet,  or  batholith,  has  an  important 
influence  on  the  texture. 

Igneous  rocks  are  characteristically  massive,  as  contrasted  with 
the  stratified  structure  of  the  sedimentary,  and  the  term  massive 
is  sometimes  employed  as  a  synonym  of  igneous.  Other  synony- 
mous terms  are  eruptive  and  anogene,  both  meaning  that  the  rocks 

15 


i6 


A   HAND  BOOK  OF  ROCKS. 


I 


I 


If, 


have  come  up  from  below.  Many 
years  ago  the  distinction  was  made 
between  those  that  have  crystallized 
deep  within  the  earth,  the  plutonic, 
and  those  that  have  been  poured  out 
on  the  surface,  the  volcanic.  The 
words  intrusive  and  effusive  or  extru- 
sive have  been  employed  in  much  the 
same  way.  Between  surface  flows  and 
deep-seated  masses  (batholiths)  and 
their  characteristic  textures,  every  gra- 
dation is  to  be  expected  and  is  met, 
and  an  intermediate  group  has  even 
been  established  by  some  writers  for 
rocks  that  have  cooled  as  intruded 
sheets  and  dikes.  This  three-fold  dis- 
tinction is  not  carried  out  here,  the  two 
extremes  being  believed  to  illustrate 
the  varieties  satisfactorily  when  accom- 
panied by  auxiliary  remarks  on  the 
intermediate  types. 

We  are  tending  more  and  more  to 
employ  the  word  structure  for  the 
larger  features  of  a  rock,  as  for  instance 
a  massive  structure  as  against  a  strati- 
fied, while  the  smaller  features  are  de- 
scribed as  textures,  as  for  instance  a 
glassy  texture,  a  porphyritic  or  a  gran- 
itoid, terms  that  refer  to  characters 
which  may  be  seen  even  on  a  small 
fragment.  Glassy  texture,  as  the  name 
implies,  is  that  of  glass  or  slag  and  has 
no  definite  minerals.  It  results  when  a 
molten  magma  is  so  quickly  chilled 
that  the  minerals  have  no  opportunity 
to  form.  Porphyritic  implies  larger 
crystals,  well  formed  or  corroded  and 
rounded,  embedded  in  a  more  finely 


IGNEOUS  ROCKS.  17 

crystalline,  or  even  in  a  "glassy  groundmass."  There  may  be  sev- 
eral sizes  and  kinds  of  these  crystals,  and  because  of  their  promi- 
nence in  the  rock  they  are  called  phenocrysts,  i.  e.t  apparent  cry- 
stals, but  phanerocryst  is  better  etymologically.  If  a  magma  cry- 
stallizes as  a  mass  of  very  fine  or  microscopic  crystals  without 
phenocrysts,  its  texture  is  described  as  felsitic.  A  granitoid  or 
granular  texture  has  the  component  crystals  all  of  about  the  same 
size,  and  very  seldom  possessing  their  own  crystal  boundaries. 
Strictly  speaking,  there  is  no  groundmass  in  granitoid  rocks. 
Sometimes  from  a  local  abundance  of  mineralizers  (as  later  ex- 
plained), granitoid  rocks  have  small  cavities  into  which  the  com- 
ponent minerals  project  with  well-bounded  crystals.  Such  are 
called  miarolitic. 

Textures  in  igneous  rock  are  due  to  several  factors  that  have  in- 
fluenced the  development  of  the  magma  during  its  consolidation. 
The  most  important  are  chemical  composition,  temperature,  rate 
of  cooling,  pressure  and  the  original  presence  of  dissolved  vapors 
called  mineralizers.  The  fusibility  varies  with  the  chemical  com- 
position. The  most  acid  or  siliceous  magmas,  i.  e.,  those  with  65— 
75  per  cent.  SiO2  are  least  fusible.  When  molten  they  are  viscid 
and  ropy.  The  fusibility  increases  with  the  decrease  of  silica  down 
to  the  basic  rocks  with  40  to  50  per  cent.  SiO2.  The  ultra-basic 
rocks  which  graduate  into  practically  pure  bases,  as  in  some  rare, 
igneous  iron  ores,  are  less  fusible.  This  statement  that  acid  rocks 
are  least  fusible  often  puzzles  a  student  who  is  familiar  with  blast 
furnace  practice  and  the  composition  of  slags,  in  which  the  most 
siliceous  are  regarded  as  most  fusible,  but  slags  themselves,  as  a 
comparison  of  analyses  will  readily  show,  are  to  be  paralleled  with 
basic  rocks.  The  importance  of  the  fusibility  as  regards  textures 
lies  in  the  fact  that  the  highly  siliceous  quickly  chill,  become  ropy 
and  freeze.  They  therefore  especially  yield  glasses.  The  easily 
fusible  remain  fluid  at  lower  temperatures,  crystallize  out  as  min- 
erals to  a  greater  degree  and  seldom  yield  glasses.  They  flow 
farther  from  the  vent  and  tend  to  develop  the  porphyritic  or  even 
a  variety  of  granular  texture.  The  influence  of  temperature  has 
been  partly  outlined  in  speaking  of  composition,  but  it  will  readily 
appear  that  in  its  progress  to  the  surface  a  basic  magma  might 
stand  for  a  considerable  period  at  a  temperature  of  fluidity,  whereas 


1 8  A   HAND  BOOK  OF  ROCKS. 

an  acid  magma  in  the  same  situation  would  consolidate.  The  rate 
of  cooling  is  important.  Cooling  magmas  tend  to  break  up  into 
minerals.  As  a  general  thing  it  requires  a  very  quick  chill  to  pre- 
vent their  formation.  Hence  it  is  that  even  volcanic  glasses  which 
appear  to  be  perfect  glass  to  the  eye  are  shown  to  be  full  of  dusty, 
microscopic  minerals  under  the  microscope.  Volcanic  glasses  are 
chiefly  found  on  the  outer  portions  of  flows  or  dikes,  but  instances 
are  known  where  sheets  of  them  are  very  thick,  as  at  Obsidian 
Cliff  in  the  Yellowstone  Park.  The  common  experience  with 
lavas  is  that  certain  crystals  develop  to  notable  size,  it  may  be  an 
inch  or  more  in  diameter,  while  the  magma  stands  beneath  the 
surface,  in  circumstances  favorable  to  their  formation.  These  are 
then  caught  up  in  the  moving  stream  and  brought  to  the  surface 
or  near  it  where  the  final  consolidation  takes  place  and  fixes  them 
in  the  so-called  groundmass.  A  quick  chill  makes  a  fine-grained 
groundmass  when  not  a  glassy  one,  and  slow  cooling  yields  one 
more  coarsely  crystalline,  but  in  the  final  cooling  or  consolidation 
at  or  near  the  surface,  crystals  are  seldom  if  ever  developed  of  a 
size  commensurable  with  those  formed  in  the  depths.  By  this 
process  of  partial  crystallization  below  and  final  consolidation  on 
the  surface,  the  porphyritic  texture  is  almost  always  developed, 
but  in  strict  accuracy  it  should  be  stated  that  cases  are  known 
where  phenocrysts  appear  to  have  formed  in  lavas  after  coming  to 
rest.  Magmas  also  flow  to  the  surface  with  no  phenocrysts  (or 
"  intratelluric  "  crystallizations)  and  then  consolidate  not  as  glass, 
but  as  finely  crystalline  aggregates,  practically  all  groundmass. 
The  resulting  texture  is  called  felsitic. 

Pressure,  such  as  is  developed  upon  a  magma  deep  within  the 
earth  or  during  its  passage  to  the  surface  is  thought  to  exert  an  in- 
fluence upon  the  formation  of  many  phenocrysts  and  to  be  necessary 
for  their  development.  Dissolved  vapors,  such  as  steam,  hydrofluoric 
and  boracic  acids,  are  also  important  factors.  Acidic  magmas  are 
more  generally  provided  with  them  than  basic,  and  where  locally 
abundant  they  lead  to  variations  both  in  the  mineral  composition  and 
texture  at  different  places  in  the  consolidated  rock.  They  may 
prevent  the  development  of  glass,  and  cause  a  sheet  such  as  Obsi- 
dian Cliff,  in  the  Yellowstone  Park,  to  present  alternations  of  glassy 
and  stony  layers,  the  latter  being  formed  of  microscopic  crystals. 


IGNEOUS  ROCKS.  19 

A  word  should  be  added  about  the  chemical  composition  of 
rocks  and  about  the  interpretation  of  analyses  before  the  rocks 
themselves  are  taken  up.  The  analyses  are  reported  in  percent- 
ages of  oxides,  for  the  most  part,  and  these  are  arranged  in  the 
following  series,  SiO2,  A12O3,  Fe2O3,  FeO,  CaO,  MgO,  Na2O,  K2O, 
H2O.  In  order  to  have  anhydrous  materials,  it  is  customary  to 
ignite  and  determine  loss  on  ignition.  This  loss  includes  both 
H2O  and  CO2  and  where  large  throws  uncertainty  over  the  relations 
of  the  elements  left  behind,  because  of  the  evident  advance  of  decay. 
Small  percentages  of  other  oxides  are  quite  invariably  present  and 
in  refined  work  are  determined.  These  are  TiO2,  MnO,  NiO, 
BaO,  SrO,  S,  Cl,  P2O5,  Li2O,  and  even  rarer  ones.  They  are  how- 
ever always  in  very  small  quantity.  We  often  recast  an  analysis, 
by  dividing,  as  in  the  determination  of  a  mineralogical  formula, 
each  percentage  by  the  molecular  weight.  We  thus  get  numerical 
molecular  ratios  which  indicate  the  relative  numbers  of  individual 
molecules  and  enable  us  to  draw  conclusions  as  to  the  way  in 
which  they  are  combined  with  one  another  in  the  component 
minerals  of  the  rock.  If  we  know  the  chemical  formulas  of  the 
minerals  we  can  sometimes  calculate  the  percentage  of  each  in  the 
rock.  In  Chapter  XIII.  this  subject  is  further  treated  with  an 
illustrative  example.  The  calculations  cannot  however  be  made 
when  two  or  more  bases  appear  together  in  two  or  more  min- 
erals. Variations  in  chemical  composition  entail  variations  in 
resulting  minerals,  but  it  is  also  true  that  the  same  magma,  if 
consolidating  under  different  physical  conditions  of  heat,  pressure, 
etc.,  at  different  times  may  yield  somewhat  different  minerals,  for 
instance,  hornblende  instead  of  augite,  or  vice  versa.  A  study  of 
analyses  soon  makes  one  more  or  less  familiar  with  the  minerals 
that  would  necessarily  result.  The  more  important  points  are  the 
amounts  of  silica,  of  the  alkalies  and  alkaline  earths,  of  iron  oxides 
and  of  alumina.  For  instance,  as  a  rule,  only  magmas  high  in 
SiO2  yield  quartz,  for  otherwise  it  would  combine  with  the  bases. 
Much  K2O  is  necessary  for  an  orthoclase  or  leucite  rock,  but  much 
Na2O  for  one  with  nepheline.  MgO  in  relatively  large  amount  is 
required  to  yield  olivine  or  an  orthorhombic  pyroxene,  and  when 
feldspars  drop  away  and  rocks  become  very  basic  we  expect  high 
CaO,  MgO,  FeO,  Fe2O3,  and  low  SiO2.  In  rocks  tested  for  pur- 


20  A  HAND  BOOK  OF  ROCKS. 

poses  of  building,  the  percentage  of  sulphur  is  important  and  very 
little  should  be  present.  It  occurs  in  some  form  of  pyrites,  which 
by  its  decay  generates  sulphuric  acid  and  destroys  the  stone  or 
stains  it  with  limonite.  It  should  never  reach  i  per  cent. 

Using  the  molecular  ratios  as  the  basis  of  plotting,  extremely  in- 
teresting and  significant  diagrams  have  been  devised,  first  for  indi- 
vidual cases  by  W.  C.  Brogger  *  and  later  of  groups  by  W.  H. 
Hobbs,f  so  that  the  latter's  figures  are  like  composite  photographs 
of  the  chemical  composition  of  the  several  groups  of  igneous 
rocks.  Diagrams  of  this  sort  are  subsequently  given  which  will 
epitomize  for  the  student  the  chemical  characteristics  of  each  of 
the  groups.  Various  other  devices  have  been  suggested  but  the 
ingenious  plan  of  Brogger  is  the  best. 

The  specific  gravity  or  density  of  a  rock  is  an  important  feature 
in  its  practical  bearings.  While  it  may  in  ice  be  less  than  I,  and 
in  coals  and  certain  carbonaceous  deposits  may  drop  as  low  as  1.25, 
and  in  very  porous  sandstones  reach  2.25,  yet  in  the  common  rocks 
it  is  seldom  below  2. 50,  and  ranges  from  this  to  over  3.00.  Granites 
are  usually  about  2.65,  but  basic  rocks,  rich  in  iron,  attain  to  the 
higher  limits,  even  above  3.0.  Determinations  are  important  in 
those  rocks  used  for  building  purposes,  and  are  expressed  in 
pounds  per  cubic  foot. 

Of  recent  years  we  have  come  to  regard  molten  magmas  as  es- 
sentially solutions  of  some  compounds  in  others,  and  to  appreciate 
that  solutions  do  not  cease  to  be  such,  even  when  the  temperature 
is  very  high.  It  results  from  this  that  the  crystallization  of  the 
minerals  of  an  igneous  rock  takes  place  from  the  magma  as  this 
in  its  cooling  successively  reaches  a  point  of  saturation  for  the  salt 
in  question.  The  least  soluble  separate  the  earliest  of  all,  and 
then  the  others  in  order ;  but  as  the  pressure  under  which  they 
rest  is  also  a  factor,  and  this  is  subject  to  variation,  as  indeed  is 
the  temperature  during  movement  to  the  surface,  one  mineral's 
period  of  formation  may  overlap  another's  more  or  less.  The 
order  of  formation  will  be  determined  by  the  laws  of  thermo- 
dynamics and  necessarily  the  mineral  that  develops  the  most  heat 
in  crystallizing  will  be  the  first  to  crystallize.  As  a  general  rule, 

*  "  Das  Ganggefolge  des  Laurdalits,"  Kristiania,  1898,  255. 

f  "  Suggestions  Regarding  the  Classification  of  the  Igneous  Rocks,"  Journal  of 
Geology,  VIII.,  i,  1900. 


IGNEOUS  ROCKS.  21 

the  relations  of  the  minerals  in  rocks  show  that  the  earliest  to  form 
are  apatite  ;  the  metallic  oxides  (magnetite,  ilmenite,  hematite) ;  the 
sulphides  (pyrite,  pyrrhotite) ;  zircon  and  titanite.  These  are  often 
called  the  group  of  the  ores.  Next  come  the  ferromagnesian  sili- 
cates, olivine,  biotite,  the  pyroxenes  and  hornblende.  Next  follow 
the  feldspars  and  feldspathoids,  nepheline  and  leucite,  but  their 
periods  often  begin  well  back  in  that  of  the  ferromagnesian  group. 
Last  of  all,  if  any  excess  of  SiO2  remains,  it  yields  quartz.  In 
the  variation  of  the  conditions  of  pressure  and  temperature  just 
referred  to,  it  may  and  does  often  happen  that  crystals  are  again 
redissolved  in  the  magma,  or  are  resorbed,  as  it  is  called ;  and  it 
may  also  happen  that  after  one  series  of  minerals,  usually  of  large 
size  and  of  intratelluric  origin,  has  formed,  the  series  is  again  re- 
peated on  a  small  scale  as  far  back  as  the  ferromagnesian  silicates. 
Minerals  of  a  so-called  second  generation  thus  result,  but  they  are 
always  much  smaller  than  the  phenocrysts,  and  are  characteristic 
of  the  groundmass. 

It  follows  from  what  has  been  stated  that  the  residual  magma  is 
increasingly  siliceous  up  to  the  final  consolidation,  for  the  earliest 
crystallizations  are  largely  pure  oxides.  It  is  also  a  striking  fact 
that  the  least  fusible  minerals,  the  feldspars  and  quartz,  are  the  last 
to  crystallize  and  therefore  we  must  introduce  the  conception  of 
solution  in  order  to  explain  the  process ;  otherwise  the  minerals 
would  inevitably  form  in  the  reverse  order  of  their  fusibilities,  the 
most  infusible  leading  off.  The  accompanying  table  of  fusing 
points  will  be  of  interest  in  this  connection.  It  must  be  borne  in 
mind  that  no  mineral  can  crystallize  while  the  magma  has  a  tem- 
perature above  its  fusing  point  for  the  conditions  of  pressure  pre- 
vailing at  the  time. 

TABLE  OF  THE  FUSING-POINTS  OF  THE  COMMON  ROCK-MAKING  MINERALS. 

^Egirite    925°  C.  Microcline  II55°C. 

Hornblende    1025    101085°     Orthoclase   , 1175 

Nephelite    1080    to  1095       Magnetite    1 185 

Augite 1085    to  1095       Hypersthene    1185 

Albite 1 1 10  Muscovite 1230 

Oligoclase 1120  Leucite    1300 

Labradorite 1125  Olivine 1350 

Biotite 1130  Bronzite 1400 

Sanidine 1 130  Quartz  fuses  still  higher. 

Anorthite    1 132 


22  A   HAND  BOOK  OF  ROCKS. 

TABLE  op  THE  FUSING- POINTS  OF  SOME  OF  THE  COMMON  IGNEOUS  ROCKS. 

Granite   1240°  C.    Basalt 1060°  C 

Monzonite 1190          Limburgite 1050 

Phonolite    1090  Lava  (Etna)  IOIO 

Lava  (Vesuvius)    1080 

These  values  are  taken  from  an  abstract  of  a  paper  by  C.  Doel- 
ter,  reviewed  in  the  Neues  Jahrbuch,  1903,  Volume  II.,  page  60. 

From  them,  it  is  evident  that  while  olivine,  for  example,  melts, 
at  1350°,  limburgite,  a  rock  containing  large  proportions  of  it,  melts 
at  1050°.  Magnetite,  the  earliest  to  crystallize  of  all  the  minerals 
mentioned,  fuses  of  itself  more  than  a  hundred  degrees  above  many 
of  the  rocks  containing  it. 

In  the  matter  of  the  study  and  determination  of  a  rock  species,  es- 
pecially of  an  igneous  rock,  it  is  desirable  to  procure  materials  as  fresh 
and  unaltered  as  possible.  If  feldspars  have  all  changed  to  kaolin 
and  clay,  and  if  ferromagnesian  silicates  are  merely  chlorite  or  ser- 
pentine, and  if  secondary  quartz,  calcite  and  the  like  have  formed,  it  is 
very  difficult  if  not  impossible  to  draw  correct  or  even  well-grounded 
inferences.  Rocks  near  ore  bodies  are  very  often  of  this  character. 

Bearing  in  mind  these  differences  of  texture  and  the  causes  of 
them,  it  is  possible  to  group  igneous  rocks  in  such  arrangement 
that  they  can  be  intelligently  studied,  and  identified  with  a  reason- 
ably close  approximation  to  the  truth.  It  should  be  appreciated, 
however,  that  with  finely  crystalline  rocks,  whose  components  are 
too  small  for  the  unassisted  eye,  the  microscope  is  the  only  re- 
source, and  with  this  as  an  aid  much  greater  subdivision  can  be 
attained.  The  object  here  in  view  is  to  limit  the  discussion  purely 
to  the  study  without  the  microscope. 

The  scheme  of  classification  of  the  igneous  rocks  has  three 
principles  underlying  it,  viz :  texture,  mineralogical  composition 
and  chemical  composition.  The  textures  are  five  :  glassy,  felsitic, 
porphyritic,  granitoid  and  fragmental,  and  the  table  is  arranged 
from  top  to  bottom  so  that  they  come  in  this  order.  The  arrange- 
ment is  adopted  because  it  brings  the  glassy  which  are  the  simplest 
of  all  rocks  at  the  outset,  where  they  can  be  best  taken  up  by  the 
beginner.  From  top  to  bottom  after  the  glassy  rocks,  the  surface 
flows  with  their  peculiar  species  come  next,  and  then  we  pass 
through  those  with  increasing  proportions  of  phenocrysts,  to  the 
thoroughly  granitoid.  The  word  porphyry  as  a  suffix  has  been 


& 


s, 
ian, 
Pitc 


ii 


il 


II 
1 


"   * 


ui 

§31 


if 


II 

It 


3U3UIUIOJJ 


it 


•oijuXndioj 


«<     0 


o    i 


24  A   HAND  BOOK  OF  ROCKS. 

adopted  for  the  intermediate  members,  which  roughly  correspond 
with  the  intrusive  rocks.  Finally  the  granitoid  batholiths  complete 
the  succession.  It  must  be  appreciated  however  that  the  methods 
of  field  occurrence  do  not  follow  these  textural  differences  in  other 
than  a  general  way.  Thus  thick  surface  flows  will  have  dense 
porphyritic  textures  at  their  centers,  and  dikes  are  known  with 
glassy  borders.  Thick,  intrusive  sheets  and  laccoliths  are  practi- 
cally granitoid,  like  the  batholiths  and  the  batholiths  themselves 
sometimes  become  roughly  porphyritic  from  the  exceptional  de- 
velopment of  the  feldspars.  But  nevertheless  an  important  general 
rule  is  emphasized  by  the  arrangement,  and  the  truth  that  texture  is 
largely  a  function  of  depth  and  pressure,  is  brought  out.  Not  all 
the  rocks  described  in  the  text  appear  in  the  table,  since  some, 
such  as  diabase,  on  which  much  stress  is  laid,  are  left  out.  All 
these  together  with  synonyms  and  relatives  will  be  subsequently 
emphasized,  so  far  as  is  appropriate  for  an  elementary  book. 

The  rocks  are  arranged  from  left  to  right  on  a  mineralogical 
principle,  and  chiefly  on  the  basis  of  the  predominant  feldspar 
present,  as  is  the  usual  custom.  This  also  makes  possible  a 
general  succession  from  those  most  acidic  on  the  left  to  those  most 
basic  on  the  right,  but  while  this  is  true  for  the  extremes  it  is  not 
strictly  so  for  intermediate  points  because  dacites  and  quartz-dio- 
rites  are  far  higher  in  silica  than  are  phonolites  and  nephelite- 
syenites,  and  even  than  trachytes  and  syenites.  The  general  range 
of  silica  is  indicated  on  the  lowest  line.  At  the  same  time  the 
importance  of  the  bases  is  not  to  be  overlooked  and  subsequent 
tables  of  analyses  are  given  so  as  to  show  the  ranges. 

The  general  and  larger  truths  of  igneous  rocks  are  fairly  well 
brought  out  in  condensed  tables  of  this  character,  even  though  ex- 
ceptional cases  are  known  which  would  require  its  modification. 
But  no  attempt  has  been  made  to  confuse  the  larger  truths  by 
mention  of  the  rarer  occurrences,  for,  as  before  stated,  only  ordinary 
examination  is  assumed  in  connection  with  this  text.  When  rare 
and  exceptional  varieties  are  met  they  should  be  placed  in  the  hands 
of  a  worker  with  a  microscope.  It  should  also  be  appreciated  in 
connection  with  the  table  that  groups  of  rocks  shade  into  one  an- 
other by  imperceptible  gradations  and  that  they  are  not  marked 
off  with  the  sharpness  of  ruled  spaces. 


CHAPTER  III. 

THE  IGNEOUS  ROCKS,  CONTINUED.     THE  GLASSES.     THE  ROCK? 

WHOSE  CHIEF  FELDSPAR  is  ORTHOCLASE.     THE  PHONO- 

LITES  AND  NEPHELITE-SYENITES. 

THE  GLASSES. 


SiO, 

AJ,O, 

Fe,0, 

FeO 

CaO 

MgO 

KSO 

N«,O 

Loss. 

Sp.  Gr. 

I. 

79-49 

II.  60 

0-33 

0.49 

1.64 

0.09 

1.52 

4.04 

0.68 

2. 

76.20 

I3-I7 

0-34 

0-73 

0.42 

0.19 

4.46 

4-31 

0-33 

2-352 

3- 

75-52 

14.11 

1.74 

0.08 

0.78 

O.IO 

3.63 

3-92 

o-39 

2.342 

4- 

74.70 

13-72 

I  OI 

O.62 

0.78 

0.14 

4.02 

3-90 

0.62 

2-345 

5- 

74-05 

13.85 

tr. 

0.90 

0.07 

4.31 

4.60 

2.  2O 

6. 

74-05 

12.97 

2-73 

0.12 

0.28 

5." 

3-88 

0.22 

2-37 

7- 

74-oi 

12-95 

1.42 

0.99 

0.48 

4.65 

5-34 

0.29 

2.391 

8. 

72.87 

12.05 

i-75 

1.30 

1.  10 

tr. 

6.13 

3.00 

9- 

71.6 

I2.O 

I.O 

I.I 

O.2 

4-3 

2-5 

7-4 

10. 

71.56 

13.10 

0.66 

0.28 

0.74 

0.14 

4.06 

3-77 

5-52 

ii. 

65-I3 

15-73 

2.24 

1.86 

3-62 

1.42 

3-96 

2-93 

2-43 

12. 

60.5 

I9.I 

4-2 

0-3 

0.6 

0.2 

3-5 

10.6 

2.48 

13- 

54.28 

14.83 

14-73 

7.02 

3-65 

1.27 

4.22 

2.704 

14. 

50.82 

9.14 

7-33 

7-03 

11.63 

7-22 

1.02 

3-o6 

i-74 

2.66 

15- 

45-73 

2O.I5 

12.46 

... 

8.67 

3-59 

4.II 

5-74 

0.  12 

I.  Pumice,  Cinder  Cone,  Calif.,  J.  S.  Diller,  Bull.  79,  U.  S.  G.  S.,  p.  29.  2.  Black 
Obsidian,  Tewan  Mtns.,  N.  M.,  J.  P.  Iddings,  7th  Ann.  Rep.  U.  S.  G.  S.,  219.  3.  Red 
Obsidian,  Yellowstone  Park,  J.  P.  Iddings,  7th  Ann.  Rep.  U.  S.  G.  S.,  219,  also  FeS, 
o.i I.  4.  Black  Obsidian,  Yellowstone  Park,  J.  P.  Iddings,  7th  Ann.  Rep.  U.  S.  G. 
S.,  219,  also  FeS,  0.40.  5.  Scoriaceous  Obsidian,  Mono  Lake,  Cal.,  I.  C.  Russell, 
8th  Ann.  Rep.  U.  S.  G.  S.,  380.  6  Obsidian,  Lipari  Is.,  Abich.  Vulk.  Ersch.,  62. 
7.  Obsidian  from  Andesite,  Clear  Lake,  Cal.,  G.  F.  Becker,  Mon.  XIII.,  U.  S.  G.  S., 
104.  8.  Perlite,  Hungary,  Kalkowsky,  Elemente  der  Lith.,  p.  75.  9.  Pitchstone, 
Meissen,  Lemberg,  Z.  d.  d.  g.  G.,  XXIX.,  508.  10.  Pitchstone,  Silver  Cliff,  Colo., 
W.  Cross,  Phil.  Soc.  Wash.,  XL,  420.  II.  Andesitic  perlite,  Eureka,  Nev.  Hague, 
Mono.  XX.,  U.  S.  G.  S.,  264.  12.  Phonolite  obsidian,  Teneriffe,  Abich.  Vulk.  Erich., 
62.  13.  Hyalomelane,  Ostheim,  Germany,  Lemberg  Z.  d.  d.  g.  G.,  XXXV.,  570. 
14.  Pele's  Hair,  Hawaii,  Cohen,  N.  J.,  1880,  II.,  41.  15.  Tachylyte,  Gethurms, 
Germany,  Lemberg,  See  No.  13. 

Comments  on  the  Analyses. — An  examination  of  the  table  of  analy- 
ses indicates  that  the  magmas  are  high  in  SiO2,  and  relatively  low 
in  all  other  bases  except  the  alkalies.  The  high  Na2O  of  Number 
12  is  worthy  of  remark,  because  this  is  the  rule  with  a  nephelite 
rock.  The  percentages  under  the  column  headed  loss,  which 

25 


26  A   HAND  BOOK  OF  ROCKS. 

practically  indicate  the  HaO  present  are  characteristic  for  different 
varieties.  They  are  low  in  the  case  of  obsidians,  Nos.  2,  3,  4,  6, 
7 ;  unusually  high  in  No.  5,  described  by  Russell  as  scoriaceous 
obsidian ;  still  higher  in  the  perlites  Nos.  8,  1 1  ;  and  reach  a  maxi- 
mum in  the  pitchstones  Nos.  9  and  10. 

Basic  glasses  are  seldom  sufficiently  free  from  included  crystals 
to  be  separable  from  the  porphyritic  rocks.  Frothy  and  cellular 
crusts  do,  however,  appear  on  lava  streams,  and  are  known  as 
scorias,  and  rare,  homogeneous  glasses  have  been  called  tachy- 
lyte  and  hyalomelane. 

Varieties.  —  The  chief  glasses  are  obsidian,  pumice,  perlite  and 
pitchstone.  The  name  obsidian  is  applied  to  homogeneous  glasses 
with  low  percentages  of  water.  The  word  is  of  classic  and  ancient 
origin  and  is  now  used  with  a  prefixed  name  for  all  glasses,  such 
as  rhyolite-obsidian,  basalt-obsidian,  etc.  Pumice  is  an  excessively 
cellular  glass,  caused  by  expanding  steam  bubbles.  Perlite  is  a 
glass  broken  into  small  onion-like,  individual  masses,  by  con- 
centric cracks,  from  contractions  in  cooling.  The  concentric, 
shelly  masses  lie  in  between  intersecting  series  of  larger,  straight 
cracks ;  the  perlites  have  considerable  water,  usually  2-4  per 
cent.  The  word  is  also  written  pearlstone,  and  was  suggested  by 
the  fancied  resemblance  of  the  concentric  shells  to  the  familiar  gem. 
Pitchstone  is  a  homogeneous  glass,  like  obsidian,  but  contains 
5-10  per  cent,  of  water.  Pitchstones  have  often  a  more  resinous 
appearance  than  obsidians,  but  there  is  no  very  essential  difference 
apparent  to  the  eye.  The  name  was  formerly  used  for  glasses  of 
earlier  geological  age  than  the  obsidians.  Obsidians  are  usually 
black  or  red,  with  translucent  edges  ;  pitchstones  are  mostly  reds 
and  greens,  but  thin  slivers  are  practically  colorless  ;  all  the  glasses 
contain  dusty,  embryonic  crystals,  gas  pores,  and  sometimes 
skeleton  crystals  of  larger  growth  and  even  a  few  phenocrysts 
which  are  often  arranged  in  flow  lines  and  swirling  eddies.  Almost 
all  large  developments  of  the  glasses  show  dense,  stony  or  lithoidal 
layers  and  streaks,  that  are  due  to  the  development  of  minute 
crystals  of  feldspar  and  quartz,  which  may  be  arranged  in  radiating 
rosettes,  called  spherulites.  The  individual  crystals  are  not  often 
large  enough  to  be  seen  with  the  unassisted  eye.  Expanded, 
bubble-like  cavities  are  also  met,  with  perhaps  several  concentric 


IGNEOUS  ROCKS.  27 

walls,  on  which  at  times  are  perched  little  well-formed  crystals. 
These  cavities  are  called  lithophysae,  i.  e.t  stone  bubbles.  Topaz, 
quartz,  tridymite,  feldspars,  fayalite  and  garnet  have  been  found  in 
beautiful  crystals  in  them.  The  lithophysae  are  due  to  the  influ- 
ence and  escape  of  mineralizers,  and  may  reach  a  diameter  of  over 
an  inch. 

Relationships.  —  The  glasses  are  all  mere  varieties  of  volcanic 
rocks,  which  a  quick  chill  has  prevented  crystallizing.  At  the 
same  time,  it  is  only  possible  by  field  associations  or  by  chemical 
analysis  to  refer  them  to  their  corresponding  porphyritic  types, 
although  in  the  great  majority  of  cases  they  are  formed  from 
rhyolitic  magmas. 

Geological  Occurrence. — The  glasses  sometimes  appear  as  inde- 
pendent sheets  and  dikes ;  more  often  they  form  the  surface  of 
well  crystallized  lava-sheets  or  the  outer  portions  of  dikes. 

Alteration.  —  Glasses  resist  alteration  notably  well,  but  in  the 
long  run  are  subject  to  decay  along  cracks  and  exposed  surfaces. 
They  yield  quartz,  kaolin  and  fine,  scaly  muscovite.  In  instances 
they  devitrify,  as  it  is  called,  or  break  up  into  aggregates  of  quartz 
and  feldspar  in  excessively  minute  crystals,  so  that  we  can  only 
trace  them  back  to  the  original  glass,  by  the  flow  lines,  spheru- 
lites,  etc.,  that  still  remain.  Such  devitrified  forms  have  been 
called  by  F.  Bascom,  apobsidian.  Petrosilex  is  an  older  term  ap- 
plied to  these  and  other  similar  rocks,  and  felsite  has  been  also 
used. 

Distribution.  —  The  glasses  are  widespread  in  the  West  Ob- 
sidian Cliff,  in  the  Yellowstone  Park,  yields  black,  red  and  stony 
varieties,  and  has  been  made  a  type  locality  by  the  studies  of  J.  P. 
Iddings.  Silver  Cliff,  Colorado,  has  furnished  some  remarkable 
pitchstones,  described  by  Whitman  Cross.  The  extinct  volcanoes 
of  New  Mexico,  Utah,  Montana  and  around  Mono  Lake,  Cali- 
fornia, are  well-known  localities.  Alaska  has  supplied  much 
from  near  Fort  Wrangel,  and  in  Mexico  and  Iceland  are  other 
prolific  sources.  Along  the  Atlantic  Coast  there  are  only  the 
devitrified  glasses  of  ancient  (pre-Cambrian)  volcanoes.  These  are 
well  developed  in  New  Brunswick,  Maine,  Massachusetts  and 
Pennsylvania.  Abroad  the  obsidian  of  the  Lipari  Islands  is  a 
famous  one,  and  the  perlites  of  Hungary  supply  the  usual  type 


28  A  HAND  BOOK  OF  ROCKS. 

specimens  in  our  collections.  The  best  known  of  all  pitchstones  are 
found  at  Meissen,  near  Dresden,  in  Saxony,  and  on  the  island  of 
Arran,  off  the  west  coast  of  Scotland. 

THE   RHYOLITE-GRANITE  SERIES. 
THE  RHYOLITES. 


SiO, 

Al.O. 

Fe,0, 

FeO 

CaO 

MgO 

K,0 

Na,O 

Loss. 

Sp.  Gr. 

I. 

78.95 

10.22 

3-23 

... 

1.84 

0.14 

1.76 

4.18 

2. 

77-5 

9-7 

6.1 

... 

... 

... 

5-8 

0-3 

0.4 

3- 

75.20 

12.96 

o-37 

0.27 

0.29 

O.I2 

8.38 

2.  02 

0.58 

4- 

73-91 

15-29 

0.89 

0.77 

4-79 

3-62 

I.I9 

5- 

73-07 

11.78 

2.30 

2.02 

o-39 

6.84 

1.19 

2.24 

6. 

71.12 

14.58 

1.69 

... 

1.50 

0.15 

6.01 

3-26 

0-95 

7- 

70.92 

13-24 

3-54 

0.66 

1.42 

0.23 

4-25 

4.28 

0-57 

8. 

70.74 

14.68 

0.69 

0.58 

4.12 

0.28 

2-59 

2.29 

2.09 

2.68 

9- 

68.84 

15-73 

... 

3-  " 

3-58 

0.90 

3-59 

2.89 

1.50 

2.4 

10. 

68.10 

14.97 

2.78 

1.  10 

3-04 

I.  10 

2-93 

3.46 

1.28 

2.636 

ii. 

67.20 

14.95 

5.19 

0.30 

2-39 

0.89 

4.00 

2.13 

12. 

66.91 

14.13 

5-00 

2-35 

0-95 

5-40 

3-86 

1.42 

13- 

66.60 

16.69 

2.06 

0-93 

1.40 

I.I5 

5-23 

2.46 

1.70 

2-43 

14. 

63-63 

17.42 

0.15 

5-76 

2.86 

... 

5-54 

4-52 

0.15 

I.  Rhyolite,  Iceland,  BackstrSm,  Contrib.  to  Icelandic  Liparites.  2.  Rhyolite, 
Wales,  A.  Harker,  Bala  Vole.  Sen,  13.  3.  Rhyolite,  Silver  Cliff,  Colo.,  Cross,  Colo. 
Sci.  Soc.,  Dec.  5,  1887,  229.  4.  Rhyolite,  Pinto  Peak,  Eureka,  Nev.,  A.  Hague, 
Mono.  XX.,  264.  5.  Rhyolite,  McClelland  Peak,  Washoe,  Dist,  Nev.,  F.  A.  Gooch, 
Bull.  17,  U.  S.  G.  S.,  33.  6.  Rhyolite,  Island  of  Ponza,  near  Naples,  quoted  by 
Kalkowsky,  Elem.  d.  Lith.,  p.  75.  7.  Rhyolite,  Yellowstone  Park,  Iddings,  Origin 
Igneous  Rocks,  Tab.  I.  8.  White  Rhyolite  Porphyry,  Leadville,  Colo.,  Cross,  Mono. 
XII.,  U.  S.  G.  S.,  326.  9.  Rhyolite,  Lassen's  Peak,  Cal.,  Fortieth  Paral.  Survey,  I., 
652.  10.  Gray  Rhyolite  Porphyry,  Leadville,  Colo.,  Mono.  XII.,  U.  S.  G.  S.,  332. 
II.  Rhyolite  Porphyry,  Flagstaff  Hill,  Colo.,  Palmer  &  Fulton,  Colo.  Sci.  Soc.,  III., 
356.  12.  Rhyolite,  Hungary,  v.  Hauer,  Verb.  d.  k.  k.  R.,  1867,  118.  13.  Rhyolite 
Porphyry,  Upper  Quinnesec  Falls,  Mich.,  G.  H.  Williams,  Bull.  62,  U.  S.  G.  S.,  1 20. 
14.  Rhyolite  Porphyry,  Waterville,  N.  H.,  G.  W.  Hawes,  N.  H.  Geol.  Surv.,  III., 
178. 

Comments  on  the  Analyses.  —  The  analyses  illustrate  the  ranges 
of  the  various  molecules.  No.  I  illustrates  the  upper  limit  of  the 
percentages  of  SiO2  and  No.  2  the  lower  limit  of  A12O3.  The 
gradual  increase  of  A12O3  in  all  the  others,  with  decrease  of  SiO2, 
and  in  general  the  same  relation  as  regards  CaO  are  worthy 
of  remark,  as  is  the  prevailingly  low  MgO.  Sometimes  K2O, 
sometimes  Na2O,  is  in  excess,  and  this  brings  out  the  reason  why 
we  speak  of  orthoclase  as  the  chief  feldspar,  not  as  the  only  one, 
in  the  table,  p.  23.  The  specific  gravity  is  in  general  low. 


IGNEOUS  ROCKS. 


29 


Fig.  8  is  a  diagram  based  upon  the  molecular  ratios  obtained 
from  those  analyses,  given  above,  which  contain  determinations 
both  of  Fe2O3  and  FeO.  Lengths  proportionate  to  the  molecular 
ratios  have  been  laid  off  upon  the  radiating  arms  and  have  then 
been  connected  by  the  bounding  lines.  As  the  molecular  ratio  of 
silica  is  much  the  largest  of  those  used  it  has  been  halved  and  the 


Si'O 


FIGS.  8  AND  9.  Diagram  illustrating  the  chemical  composition  of  the  rhyolites  whose 
analyses  are  given  in  the  above  table.  Fig.  8  is  based  on  molecular  ratios  ;  Fig.  9  on 
percentages. 

halves  have  been  laid  off  on  the  horizontal  line,  each  way  from  the 
middle  point.  The  resulting  figure,  with  its  long  arms  for  silica, 
its  quite  long  arms  for  alumina  and  the  alkalies,  and  its  short  ones 
for  lime,  iron  and  magnesia,  is  very  characteristic  and  its  signifi- 
cance will  be  the  more  evident  when  compared  with  the  diagrams 
which  follow.  Fig.  9  has  been  drawn  in  the  same  way  but  the 
percentages  obtained  by  averaging  the  same  analyses  which  were 
used  for  the  molecular  ratios  in  Fig.  8  have  been  employed.  In  a 
general  way  the  figures  resemble  each  other,  but  those  oxides 
whose  molecular  weights  are  high,  such  as  iron,  have  relatively 
shorter  arms  in  Fig.  8  than  in  Fig.  9.  Fig.  9  is  given  so  as  to 
accustom  the  student  to  pass  from  percentages,  with  which  he  is 
familiar,  to  molecular  ratios  to  which  he  is  usually  relatively  unac- 
customed ;  and  yet  as  explained  on  page  19  the  latter  are  the  more 
significant  in  the  chemistry  of  rocks. 


30  A  HAND  BOOK  OF  ROCKS. 

General  Description. — The  Rhyolite  Series  embraces  a  large  and 
diversified  group  of  rocks.  All  its  members  have  the  light-colored 
minerals,  quartz,  orthoclase  and  plagioclase  in  great  excess.  The 
dark-colored  minerals,  biotite,  hornblende  and  augite,  of  which 
biotite  is  the  commonest,  are  greatly  in  the  minority.  This  is  most 
emphatically  shown  in  the  more  acidic  members,  but  while  always 
pronounced  the  disparity  is  less  marked  in  those  with  the  lower 
percentages  of  silica.  The  accessory  minerals,  magnetite,  hema- 
tite, pyrite,  etc.,  are  few  and  inconspicuous.  The  prevailing  colors 
are  light  grays,  yellows  and  pale  reds,  but  darker  shades  especially 
of  red  are  not  uncommon.  The  rhyolites  have  high  fusing  points 
ranging  above  1200°  C.  (2200°  F.).  When  molten  they  are 
therefore  usually  viscous  and  thick  and  their  movements  are  not 
marked  by  the  fluidity  shown  by  the  more  basic  rocks.  Hence, 
when  solidified  they  often  exhibit  the  flow  lines  still  preserved, 
which  originally  suggested  their  name  from  the  Greek  verb  mean- 
ing to  flow. 

The  textures  of  the  rhyolite  series  vary  widely  and  upon  them 
are  based  the  principal  varieties.  The  Rhyolites  proper  are  felsitic 
or  moderately  porphyritic  rocks,  often  somewhat  cellular  because 
of  their  typical  occurrence  in  surface-flows,  whose  dissolved  vapors 
have  expanded  under  the  comparatively  slight  pressure  of  the 
atmosphere  and  have  caused  the  cavities.  Spherulites,  lithophysae 
and  the  minerals  which  are  characteristic  of  the  latter  are  often 
found  in  rhyolites.  When  the  texture  is  felsitic  it  may  be  impos- 
sible to  distinguish  and  recognize  the  small  component  minerals, 
even  with  a  lense,  and  then  the  rocks  must  be  identified  by  their 
light  color  and  specific  gravity.  Such  rhyolites  are  sometimes 
described  as  lithoidal,  and  the  microscope  has  shown  that  their 
component  minerals  are  minute  quartzes  and  feldspars,  often  with 
some  glass.  If  in  doubt  as  between  rhyolites  and  trachytes  or 
dacites  the  non-committal  term  felsite  is  then  often  convenient. 
With  the  development  of  phenocrysts  the  exact  determination  of 
the  rock  becomes  less  difficult.  Quartz  and  the  feldspars  are  the 
prominent  ones,  the  dark  silicates  often  being  scarcely  apparent. 
The  groundmass  is  usually  felsitic  but  it  may  be  glassy.  The 
phenocrysts  exhibit  their  characteristic  crystal  forms  unless  rounded 
by  corrosion.  The  quartzes  are  double  six-sided  pyramids,  almost 
never  with  a  visible  prism. 


IGNEOUS  ROCKS.  31 

The  Rhyolite  Porphyries  have  abundant  phenocrysts.  The  cellu- 
lar texture  disappears  and  dense  felsitic  groundmasses  are  the  rule. 
The  phenocrysts  in  typical  cases  make  up  about  half  the  rock. 
These  textures  are  characteristic  of  the  central  portions  of  thick 
surface  flows,  and  of  dikes,  intruded  sheets  and  the  outer  parts  of 
laccoliths. 

The  Granite  Porphyries  result  when  the  phenocrysts  are  in 
marked  excess  over  the  groundmass  and  constitute  the  greater 
part  of  the  rock.  The  groundmass  is  felsitic  but  becomes  in- 
creasingly coarsely  crystalline,  as  these  rocks  approximate  the 
granites.  The  granite  porphyries  occur  in  deep-seated  dikes,  thick 
intruded  sheets  and  the  central  portions  of  laccoliths.  They  mark 
a  textural  transition  to  the  granites. 

Synonyms  and  Relatives. — The  name  rhyolite  was  first  given  by 
von  Richthofen  in  1860  to  the  rocks  which  had  previously  been 
called  quartz-trachytes.  About  a  year  afterward  Justus  Roth  sug- 
gested liparite  for  the  same  group,  a  name  derived  from  the  Lipari 
Islands  between  Naples  and  Sicily,  where  these  rocks  are  char- 
acteristically developed,  and  liparite  in  consequence  is  much  used 
by  European  geologists.  When  both  these  names  were  applied, 
and  first  used,  they  were  intended  for  Tertiary  and  later  eruptives 
alone.  The  pre-Tertiary  representatives  were  called  quartz-por- 
phyries. With  the  disappearance  of  this  time-distinction  quartz- 
porphyry  became  restricted  to  the  intrusive  dikes  and  sheets  with 
their  dense  textures  as  contrasted  with  the  rhyolites  proper  or  sur- 
face flows.  It  is  practically  a  synonym  of  rhyolite-porphyry  as  used 
here,  which  is  also  a  term  long  current,  the  latter  expression  ad- 
mitting as  it  does  of  analogous  and  uniform  names  all  through  the 
series  of  igneous  rocks.  A  synonym  of  granite-porphyry  as  here 
used  is  nevadite,  a  word  suggested  by  von  Richthofen  for  those 
rhyolites  with  an  excess  of  phenocrysts.  Recrystallized  and 
usually  more  or  less  silicified  rhyolites  which  have  suffered  meta- 
morphism  in  the  long  course  of  geologic  time  have  been  called  by 
F.  Bascom,  aporhyolites. 

Certain  close  relatives  of  the  rhyolite  series  which  are  rich  in 
soda  and  whose  feldspar  is  thus  anorthoclase  have  been  called 
quartz-pantellerites  from  the  island  of  Pantelleria  in  the  Mediter- 
anean,  off  the  coast  of  Spain,  and  their  pre-Tertiary  equivalents 


32  A   HAND  BOOK  OF  ROCKS. 

quartz-keratophyrs.  They  cannot  be  distinguished  from  the  rhyo- 
lite  series  without  the  microscope.  Fuller  details  are  given  in  the 
Glossary,  where  will  also  be  found  grorudite,  paisanite  and  several 
others. 

Relationships.  —  Rhyolites  pass  by  insensible  gradations  into 
glasses  on  one  side,  trachytes  on  another,  granites  on  a  third 
and  dacites  on  a  fourth.  Without  the  microscope  rhyolites 
can  only  be  identified  with  certainty  by  recognizing  the  quartz, 
and  may  then  be  confused  with  dacites.  The  striated  feld- 
spar of  the  latter  is  our  chief  means  of  distinction  between  the 
two. 

Alteration. — Ordinary  decay  leads  to  the  formation  of  clays  and 
kaolin.  In  metamorphic  alterations  the  rhyolites  pass  into  very 
finely  crystalline  aggregates  of  quartz  and  feldspar,  and  then  it  is 
difficult  to  decide  what  minerals  are  original  and  what  secondary, 
and  whether  the  original  rock  was  a  massive  one  or  a  tuff.  Shear- 
ing stresses  develop  schistose  structures,  and  when  decay  is  further 
superadded,  sericite  schists  may  result  that  are  extremely  difficult 
geological  problems. 

Distribution.  —  Rhyolites  are  common  in  the  Western  States, 
being  well  known  in  the  Black  Hills :  the  Yellowstone  Park ;  in 
Colorado,  where  Chalk  Mountain,  near  Leadville,  is  a  type  locality 
for  nevadite  (granite  porphyry)  ;  in  Nevada,  both  near  Eureka  and 
near  the  Comstock  lode,  and  in  California.  The  rhyolite-por- 
phyries  have  been  met  in  many  Western  districts,  but  are  of  es- 
pecial importance  at  Leadville,  where  they  are  intimately  associ- 
ated with  the  ores.  The  ancient  rhyolite-porphyries  have  also  an 
important  development  on  Lake  Superior.  The  greater  part  of  the 
boulders  in  the  Calumet  copper-bearing  conglomerate  consists  of 
them,  and  Lighthouse  Point,  near  Marquette,  furnishes  an  outcrop. 
Along  the  Atlantic  Coast  the  pre-Cambrian  rhyolites  (felsites)  are 
present  in  the  same  localities  as  those  cited  for  volcanic  glasses. 
Recent  rhyolites  are  in  vast  quantity  in  Iceland.  Many  are  known 
in  Europe,  but  the  enormous  development  in  Hungary  is  especially 
worthy  of  note.  The  sheets  of  rhyolite  on  the  Lipari  Islands  be- 
tween Naples  and  Sicily  suggested  the  name  liparite.  In  almost 
all  volcanic  districts  they  are  liable  to  occur.  In  the  Tyrolese 
Alps  rhyolite-porphyries  are  of  great  extent,  and  in  Scandinavia 


IGNEOUS  ROCKS.  33 

and  in  Cornwall,  they  form  important  dikes,  familiar  to  all  students 
of  the  subject. 

RHYOLITE  TUFFS. — These  are  the  fragmental  ejectamenta  from 
explosive  eruptions  that  often  afford  very  extensive  strata  of  rock. 
Although  loose  at  the  time  of  falling,  they  may  become  consoli- 
dated in  the  course  of  time,  or  before  this  occurs  they  may  be  sorted 
and  redeposited  in  water  so  as  to  share  the  nature  of  a  true  sediment. 
Fragments  of  volcanic  glass  and  of  all  the  component  minerals  of 
rhyolite  make  them  up,  while  larger  fragments  of  rock  and  vol- 
canic bombs  are  at  times  intermingled.  Tuffs  of  ancient  geolog- 
ical date  become  metamorphosed  and  recrystallized,  so  as  to  afford 
products  not  to  be  easily  distinguished  from  compact  felsites. 

Rhyolite  tuffs  are  abundant  along  the  eastern  foothills  of  the 
Front  Range  of  Colorado,  and  are  extensively  quarried  for  a  rather 
soft,  building  stone. 

THE  GRANITES. 

SiO,         A1,OS        Fe.O,      FeO         CaO         MgO         K,O       Na,O       Lois       Sp.  Gr. 

i-        73-76      13-43  !-i6  1.42      0.75      5.22      4.01      0.42      2.63 

2.  73.70       14.44      °-43       1-49       1-08        tr.        4.43      4.20      0.40      2.69 

3.  73.05       14.53      2.96        2.06        tr.        5.39       1.73      0.29 

4.  72.73  16.95  I-°5  tr-  8.15         0.90        O.22 

5.  72.26  13.59  1.16  2.18  1.13  0.06  5.58  3.85  0.47      2.65 

6.  7178  14.75       1.94  2.36  0.71  4.89  3.12  052 

7.  71.64  15.66  2.34        2.70  tr.  5.60  1.58  0.48 

8.  69.46  17.50  2.30        2.57  0.30  4.07  2.93  0.82      2.687 

9.  69.28  17.44  2-3°        2.30  0.27  2.76  3.64 

10.  68.68  16.28  0.66  2.55  2.24  0.81  4.07  2.88  0.85 

11.  66.84  18.32  2.27  0.20  3.31  0.81  2.80  5.14  0.46 

12.  66.68  14.93  l-S%  323  4-89  2.19  2.05  2.65  1.25 

13.  66.40  17.13        3.77  .  c5  0.94  2.08  4.49  1.03 

I.  Biotite  granite,  Green's  Landing,  Me.,  E.  F.  Hicks.  Privately  communicated. 
2.  Granitite,  Peterhead,  Scotland,  Phillips,  Q.  J.  G.  S.,  XXXVI.,  1880,  13.  3.  Red 
Granitite,  Westerly,  R.  I.,  F.  W.  Love,  for  J.  F.  K.,  Bull.  Geol.  Soc.  Amer.,  X.,  375. 
4.  Red  Granite,  Stony  Point,  Conn.,  L.  P.  Kinnicut,  Anal.,  Idem.  5.  Albany  granite, 
N.  H.,  Hornblende  granite,  G.  W.  Hawes,  A.  J.  S.,  iii.,  XXL,  25.  6.  Hornblende 
granite  with  biotite,  Cottonwood  Canon,  Utah,  T.  M.  Drown,  4Oth  Parallel  Surv.,  I., 
no.  7.  Gray  granitite,  Westerly,  R.  L,  see  No.  3.  8.  Typical  granite,  Chester, 
Mass.,  L.  M.  Dennis,  for  J.  F.  K.,  N.  Y.  Acad.  Sci.,  XL,  129.  9.  Biotite  granite, 
Raleigh,  N.  C.,  G.  P.  Merrill,  Stones  for  building  and  decoration,  418.  IO.  Biotite 
granite  with  hornblende,  Wood  Cone,  Eureka  Dist.,  Nev.,  Arnold  Hague,  Mono. 
XX.,  U.  S.  G.  S.,  228.  n.  Augite-soda  granite,  Kekequabic  Lake,  Minn.,  U.  S. 
Grant,  Amer.  Geol.,  June,  1893,  385.  12.  Granitite,  Rowland ville,  Md.,  G.  P. 
Grimsley,  Jour.  Cin.  Soc.  Nat.  His.,  1894,  p.  32.  13.  Biotite  granite  with  horn- 
blende,  El  Capitan,  Yosemite,  see  No.  6. 
3 


34 


A  HAND  BOOK  OF  ROCKS. 


Comments  on  the  Analyses.  —  These  analyses  illustrate  the  gen- 
eral range  of  SiO2,  but  granites  are  known  outside  of  both  limits. 
As  SiO,  decreases  the  bases  increase,  and  soda  tends  to  exceed 
potash,  marking  the  passage  to  the  diorites.  Those  high  in  Na2O, 
like  No.  n,  are  often  called  soda-granites.  They  are  analogous 
to  the  keratophyres,  soda-rhyolites  and  pantellerites,  earlier  re- 
ferred to.  The  whole  table  is  a  close  parallel  to  that  of  the 
rhyolites.  The  analyses  are  selected,  so  far  as  possible,  to  repre- 
sent prominent  building  stones. 


SiO, 


FIGS.  10  AND  ii.  Diagrams  illustrating  the  chemical  composition  of  the  granites, 
given  in  the  above  table.  Fig.  10  is  based  on  molecular  ratios  ;  Fig.  1 1  on  percentages. 

Figs.  10  and  1 1  have  been  drawn  upon  the  basis  of  the  average 
molecular  ratios  and  percentages  of  the  above  analyses  as  was 
explained  under  the  rhyolites.  The  figures  are  practically  the 
same  as  those  for  the  rhyolites. 

Mineralogical  Composition  and  Varieties.  —  Granites  are,  preemi- 
nently, granitoid  rocks  consisting  of  orthoclase,  sometimes  micro- 
cline,  some  acid  plagioclase,  quartz,  and  in  the  typical  variety  both 
biotite  and  muscovite.  The  light  colored  minerals  are  in  marked 
excess.  Magnetite,  apatite  and  zircon  are  always  present,  though 
small,  and  garnet  is  not  at  all  unusual.  Biotite  is  much  the  com- 
moner of  the  micas,  and  when  it  is  present  alone  the  rock  is  some- 
times called  granitite.  Granites  with  muscovite  alone  are  especially 
found  in  the  form  of  dikes.  They  are  called  aplite.  Hornblende  is, 


IGNEOUS  ROCKS.  35 

also  frequently  met,  either  with  biotite  or  by  itself,  giving  then  horn- 
blende-granite. In  former  years  this  aggregate  was  called  syenite, 
but  the  modern  usage  is  different.  Augite  in  granites  is  uncom- 
mon, and  marks  a  passage  to  the  gabbros.  All  forms  of  dark 
silicates  and  mica  may  fail,  and  then  we  have  the  so-called  binary 
granites.  Some  Missouri  granites  are  of  this  character. 

Especially  in  regions  of  granite  intrusions  and  of  extensive  meta- 
morphism,  veins  or  dikes  —  it  is  an  open  question  which  is  the 
more  correct  term  —  are  met,  which  are  formed  of  very  coarsely 
crystalline  aggregates  of  the  same  minerals  that  constitute  granite. 
These  are  called  pegmatite  and  in  them  is  the  home  of  graphic 
granite,  the  curious  intergrowth  of  quartz  and  feldspar,  such  that 
a  crobs  fracture  of  the  blades  of  quartz  suggests  cuneiform  char- 
acters. Garnet,  tourmaline,  beryl  and  minerals  involving  the  rare 
earths,  are  often  found  in  pegmatites,  and  they  supply  the  feldspar 
and  mica  of  commerce.  The  outcrops  may  be  two  hundred  feet 
broad  or  more,  and  again  the  same  aggregates  are  found  as  small 
lenses  or  "  Augen "  in  metamorphic  rocks.  In  regard  to  the 
larger  veins  or  dikes  it  seems  improbable  that  true  igneous  fusion 
could  have  afforded  such  coarsely  crystalline  aggregates,  and  so 
we  are  forced  to  assume  such  abundance  of  steam  and  other 
vapors,  i.  e.,  mineralizers,  as  to  almost,  if  not  quite,  imply  solution. 

The  fusing  point  of  granite  has  been  determined  at  about  2250° 
F.  (1240°  C),  but  it  would  of  course  vary  with  the  composition, 
being  highest  in  those  richest  in  silica. 

The  outer  portions  of  granite  masses  are  often  subjected  to  the 
action  of  escaping  vapors,  containing  boracic  and  hydrofluoric  acids. 
These  develop  tourmaline  in  quantity  and  often  fluorite,  and  in  rare 
instances  cassiterite.  In  a  famous  case  near  Luxullian,  in  Corn- 
wall, the  feldspar  has  become  changed  to  an  aggregate  of  tourmaline 
needles  and  quartz,  and  the  rock  is  called  luxullianite.  Tourma- 
line granite  is,  however,  also  known  in  which  tourmaline  plays  the 
role  of  mica  or  hornblende,  as  at  Predazzo,  in  the  Tyrol.  The 
vapor  may  change  the  borders  of  granites  to  a  mass  of  quartz  and 
a  lithia  mica,  affording  the  rock  that  is  called  greisen  and  that  is  a 
familiar  gangue  for  tin  ores. 

Granites  are  commonly  gray,  bluish  or  reddish  in  color.  The 
feldspar  is  mainly  responsible  for  this,  as  quartz  is  colorless  and 


36  A   HAND  BOOK  OF  ROCKS. 

transparent  and  biotite  and  hornblende  are  not  specially  abundant ; 
but  unusual  richness  in  the  last  named  silicates  tends  to  darken  the 
shade.  These  latter  are  very  frequently  segregated  into  the  black 
bunches  that  are  noticeable  in  many  building  stones.  The  dark 
minerals  may  assume  concentric  layers,  affording  so-called  orbicu- 
lar granite. 

Relations /tips.  —  The  passage  of  granites,  through  granite-por- 
phyries and  micro-granites,  into  rhyolite-porphyries  and  felsites,  has 
been  remarked.  Sometimes  along  the  border  of  an  intrusion,  this 
can  be  traced  inch  by  inch  to  a  place  where  the  porphyritic  texture 
is  due  to  a  quick  chill.  Mt.  Willard,  in  the  Crawford  Notch  of  the 
White  Mountains  is  a  classic  locality  for  this  transition.  It  was 
described  in  1881  by  Geo.  W.  Hawes  (see  analysis  6),  and  will  be 
referred  to  again  under  the  products  of  contact  metamorphism.  The 
close  relationship  of  the  granite  porphyries  or  nevadites  with 
granite  need  only  to  be  referred  to.  As  quartz  decreases,  syen- 
ites result  by  insensible  gradations,  and  as  hornblende  or  bio- 
tite and  plagioclase  increase,  the  same  passage  is  made  to  dior- 
ites.  Intermediate  varieties,  which  are  very  common,  are  often 
called  granite-diorites  or  grano-diorites.  Transitional  passages 
to  gabbro,  from  increase  of  augite  and  plagioclase,  are  also  well 
recognized. 

Geological  Occurrence.  —  Granites  in  their  most  typical  devel- 
opment constitute  great  irregular  masses  that  have  solidified  at 
depths ;  such  are  called  batholiths,  and  it  is  generally  believed  that 
before  consolidating  they  have  often  fused  their  way  upward  by 
melting  into  themselves  overlying  rock.  Granites  also  appear  as 
irregular  or  rounded  outcrops  in  the  midst  of  other  rocks  (bosses 
or  knobs)  and  as  dikes.  There  is  no  reason  why  granites  should 
not  form  at  all  geological  ages,  but  those  open  to  our  observation 
are  mostly  Archean  and  Paleozoic  because,  being  deep-seated  rocks, 
only  the  older  ones  have  been  exposed  by  erosion.  The  relations 
of  pegmatites  to  veins  have  been  earlier  set  forth.  Granites  tend  to 
break  apart  along  jointing  planes  into  rectangular  blocks,  a  property 
that  much  facilitates  their  quarrying.  They  also  have  lines  of  weak- 
ness admitting  of  their  further  division  into  smaller  masses.  The 
development  of  these  is  more  or  less  characteristic  of  each  particular 
locality. 


IGNEOUS  ROCKS.  37 

Uses.  —  Granites  are  much  more  extensively  employed  for  struc- 
tural purposes  than  any  other  igneous  rock,  and  indeed  in  the  trade 
any  crystalline  rock  consisting  of  silicates  is  called  granite.  They 
are  in  general  the  strongest  of  the  common  building  stones.  Crush- 
ing resistances  range  from  10,000  to  25,000  pounds  per  square  inch 
in  a  2-inch  cube.  The  important  points  are  homogeneity  of  texture, 
good,  rectangular  cleavages  in  the  quarry,  adaptability  to  tool  treat- 
ment, durability  and  pleasing  color. 

Alteration,  Metamorphism.  —  In  ordinary  decay  granites  suffer 
first  by  the  oxidation  of  the  protoxide  of  iron  in  the  ferromagnesian 
silicates  (biotite,  hornblende),  and  the  formation  of  chlorite  and 
other  secondary  minerals.  The  feldspars  also  kaolinize,  and  the 
rock  thus  becomes  hydrated.  Pyrite,  if  present,  is  an  active  agent 
in  decay.  Yet  the  chemical  changes  involved,  except  hydration, 
seem  to  be  comparatively  slight  even  in  the  change  from  granite 
to  soil.  G.  P.  Merrill  gives  the  following  analyses  of  unaltered 
and  altered  biotite  granite  from  the  vicinity  of  Washington,  D.  C. 
(Bull.  Geol.  Soc.  Amer.,  VI.,  323): 

SiO,  Al,0,          Fe.O,         FeO  C«O  MgO  K4O         Na.O         Ignition. 

1.  69.33         *4-33          —          3-6o        3.21         2.44        2.67        2.70  1.22 

2.  66.82        15.62        1.88        1.69        3.13        2.76        2.04        2.58  3.27 

3.  65.69  15.23  4.39  2.63  2.64          2.00          2.12  4.70 

No.  I  is  fresh  and  undecomposed  rock ;  No.  2,  decomposed  but 
still  moderately  firm  rock  ;  No.  3,  soil.  It  is  evident  at  once  that 
there  has  been  considerable  hydration,  and  that  a  notable  decrease 
in  the  alkalies  has  occurred,  each  being  affected  about  equally  in 
the  end,  although  K2O  yields  first ;  MgO  has  relatively  increased  ; 
CaO  has  suffered  loss  ;  the  FeO  is  all  oxidized,  the  A12O3  has  rela- 
tively increased  and  the  SiO2  decreased.  While  appreciating  these 
chemical  changes,  Dr.  Merrill  still  emphasizes  the  much  greater 
importance  of  the  physical  alteration  and  attributes  this  to  swelling 
from  hydration.  Other  interesting  data  are  given  in  the  citation. 
Similar  sets  of  parallel  analyses  have  been  made  abroad  with 
analogous  results  in  the  case  of  the  chemical  rearrangements. 

Under  dynamic  stress  granites  are  more  or  less  crushed  and 
have  their  minerals  drawn  out  into  laminations  from  shearing 
strains  so  that  they  readily  assume  gneissoid  structures.  Beyond 
question  many  gneisses  have  resulted  in  this  way,  and  in  the  geol- 


38  A  HAND  BOOK  OF  ROCKS. 

ogy  of  some  districts,  as,  for  instance,  the  Front  Range  of  Col- 
orado, we  employ  the  term  granite-gneiss.  The  structures  were, 
doubtless,  induced  while  the  granite  was  deeply  buried  and  sub- 
jected to  pressure  when  closely  confined,  so  that  the  yielding  came 
in  a  gradual  flow. 

Distribution.  —  Granites  are  abundant  along  the  Atlantic  coast, 
and  are  near  tidewater  from  Canada  to  Virginia.  Farther  south 
they  lie  back  of  the  Coastal  Plain.  They  are  chiefly  biotite  granite 
and  are  extensively  quarried.  A  famous  hornblende  granite  is 
obtained  at  Quincy,  Mass.,  and  was  formerly  called  syenite.  In 
the  old  crystalline  areas  of  Michigan,  Wisconsin  and  Minnesota 
they  are  common.  Missouri  has  many  in  the  region  of  the  por- 
phyries, already  cited.  In  the  West,  the  Black  Hills,  the  Rocky 
Mountains,  the  Wasatch  and  the  Sierras  are  abundantly  supplied. 
They  are  equally  common  in  Europe  and  elsewhere  the  world  over. 

T/te  Rhyolite-Granite  Table.  —  In  the  following  table  and  the 
three  other  similar  ones  which  later  appear,  representative  analyses 
have  been  selected  from  those  used  in  the  descriptive  text.  They 
have  then  been  recast  according  to  the  methods  described  under 
Chapter  XIII.,  so  as  to  obtain  the  percentage  by  weight  of  the 
component  minerals  of  the  rock.  These  are  given  under  the 
column  Wt.  The  percentages  by  weight  have  then  been  divided 
by  the  respective  specific  gravities  of  the  several  minerals,  the 
quotients  being  proportional  to  the  volumes  of  the  minerals  in  the 
rock.  The  quotients  are  then  reduced  to  percentages  so  that 
under  the  column  headed  Vol.  we  see  the  proportion  of  the  rock 
mass  made  up  of  each  mineral.  These  values  are  of  especial 
significance  in  connection  with  sight  determinations.  Necessarily 
minerals  with  low  specific  gravities  have,  as  compared  with  per- 
centages by  weight,  relatively  increased  percentages  by  volume ; 
whereas  minerals  of  high  specific  gravities  relatively  decrease. 
Contrasted  proportions  of  light-colored  and  dark -colored  minerals 
are  also  given  since  they  are  all-important  in  the  general  impres- 
sion given  by  a  rock.  The  calculations  unavoidably  involve 
assumptions  at  times  but  are  on  the  whole,  correctly  representa- 
tive. As  explained  under  Chapter  XIII.,  difficulties  arise  where  an 
oxide  appears  in  two  different  minerals  as  when  K2O  is  in  both 
orthoclasc  and  biotite. 


IGNEOUS  ROCKS. 


39 


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40  A  HAND  BOOK  OF  ROCKS. 

Study  of  the  Rhyolite-Granite  table  on  p.  39  shows  that  the 
light-colored  minerals  are  greatly  in  excess.  The  maximum  per- 
centage by  volume  is  98.9 ;  the  minimum  87.9.  We  may  say 
therefore  that  rarely  are  quartz  and  feldspar  less  than  nine  tenths 
ot  the  volume  of  these  rocks.  Quartz  ranges  from  a  maximum 
of  30.5  to  a  minimum  of  6.4,  but  is  in  general  over  20  per  cent. 
Orthoclase  has  a  maximum  of  49.1  but  with  relative  increase  of 
albite  (i.  e.,  of  soda)  it  may  drop  to  14.5.  In  the  last-named  case 
albite  reaches  its  maximum  of  44.2  —  and  in  the  former  its  mini- 
mum of  16.5.  Anorthite  rules  low,  1.3  as  a  minimum,  with  an 
exceptional  maximum  of  15.8.  Biotite  has  a  minimum  of  .9  and 
a  maximum  of  7.3.  Amphibole  ranges  from  2  to  10.  Magnetite 
is  usually  less  than  I,  but  has  a  maximum  of  2.  Although  ortho- 
clase  yields  at  times  to  albite,  the  alkaline  feldspars  are  always 
in  great  excess  over  the  lime-soda  varieties. 

THE  TRACHYTE-SYENITE  SERIES. 
THE  TRACHYTES. 

SiO,  A1,O,        Fe,O,       FeO        CaO         MgO        K,O       Na,O       Loss.     Sp.Gr. 


I. 

66.03 

18.49 

2.18 

0.22 

0.96 

0-39 

5.86 

5-22 

0.85 

2.59 

2. 

65.07 

16.13 

5.17 

... 

2.74 

0.67 

4-44 

4-77 

0.70 

3- 

62.28 

19.17 

3-39 

... 

1.44 

... 

5-93 

5-37 

2-33 

2.65 

4- 

62.17 

18.58 

2.15 

1.05 

1-57 

0-73 

3-88 

7.56 

1.70 

5- 

58.70 

19.26 

3-37 

0.58 

1.41 

0.76 

4-53 

8.55 

2.64 

6. 

57-7 

17.9 

4-4 

3-9 

3-7 

1.8 

7-7 

3-8 

O.I 

2.61 

I.  Trachyte,  Game  Ridge,  Custer  Co.,  Col.,  Cross,  Proc.  Col.  Sci.  Soc.,  1887, 
237.  2.  Oligoclase- trachyte,  Drachenfels  on  Rhine,  Rammelsberg,  Z.  d.  d.  g.  G., 
XI.,  440.  1859.  3.  So-called  Bostonite  dike,  Lake  Champlain,  J.  F.  Kemp,  Bull. 
107,  U.  S.  G.  S.,  20.  4-5.  Acmite-trachyte,  Crazy  Mountains,  Mont.,  Wolff  and 
Tarr,  Bull.  Mus.  Comp.  Zo5l.,  XVI.,  232.  6.  Trachyte,  Arso  Flow,  Ischia  near 
Naples.  Abich,  Isola  d' Ischia,  38.  Silica  determinations  on  eleven  trachytes  from 
the  Black  Hills  afforded  J.  H.  Caswell  values  from  65.46  to  52.02. 

Comments  on  the  Analyses.  — The  decrease  in  silica  and  the  in- 
crease in  alumina  and  the  alkalies  as  against  the  rhyolites  are  note- 
worthy. The  alkalies  in  particular  are  high,  with  sometimes  pot- 
ash, sometimes  soda,  in  excess.  The  latter  marks  the  passage  to 
the  phonolites. 

Figs.  1 2  and  1 3  have  been  drawn  respectively  upon  the  basis  of 
the  average  molecular  ratios  and  of  the  average  percentages  of  the 
above  analyses.  As  compared  with  the  figures  of  the  rhyolite- 


IGNEOUS  ROCKS.  41 

granite  series  they  show  shortened  intercepts  for  silica,  lengthened 
ones  for  alumina  and  the  alkalies,  and  to  a  less  degree  for  the  other 
components. 

General  Description.  —  The  Trachyte  Series  embraces  a  group 
of  rocks  of  considerable  diversity.  All  its  members  have  the  light- 
colored  minerals  in  excess.  The  feldspars  are  much  the  most 
prominent  components  and  give  the  pronounced  character  to  the 
rock.  Quartz  practically  fails  although  an  occasional  crystal  may 
be  seen.  The  passage  from  rhyolites  to  trachytes  marks  its  disap- 


SJd 


FIGS.  12  AND  13.  Diagrams  illustrating  the  chemical  composition  of  the  trachytes 
which  appear  in  the  above  table.  Fig.  12  is  based  on  molecular  ratios ;  Fig.  13  on 
percentages. 

pearance.  Biotite  is  on  the  whole  the  most  common  of  the  dark 
silicates,  but  both  hornblende  and  augite  are  well  known.  As 
with  the  rhyolites  the  prevailing  colors  are  light  grays,  yellows  and 
pale  reds,  with  occasional  darker  shades.  The  trachytes  have 
somewhat  lower  fusing  points  than  the  rhyolites,  ranging  some- 
where about  2000°  F.  (1100°  C.)  and  above.  They  therefore 
afford  glasses  much  less  often  and  less  readily  than  the  rhyolites, 
and  show  a  greater  tendency  to  appear  as  thoroughly  crystalline 
rocks. 

The  textures  of  the  trachyte  series  range  from  felsitic  to  coarsely 
porphyritic.  The  Trachytes  proper  are  felsitic  or  moderately  por- 


42  A   HAND  BOOK  OF  ROCKS. 

phyritic  rocks,  sometimes  cellular  from  their  crystallization  as  sur- 
face flows.  When  finely  felsitic  they  cannot  readily  be  distin- 
guished from  rhyolites,  dacites  and  andesites  without  microscopic 
examination,  and  then  felsite  is  the  only  name  which  can  safely  be 
applied  to  them  by  the  observer,  Under  the  microscope  the  felsitic 
mass,  whether  forming  the  entire  rock  or  only  the  groundmass, 
is  very  often  found  to  be  an  aggregate  of  little  rods  of  orthoclase 
arranged  in  more  or  less  flowing  lines  and  with  their  long  dimen- 
sions parallel  with  one  another.  As  groundmasses  become 
coarser  this  peculiar  texture  can  sometimes  be  detected  with  the 
eye  alone  and  even  in  fairly  coarse  syenites  it  may  be  recognizable. 
It  is  characteristically  known  as  the  "  trachyte  texture." 

With  the  development  of  phenocrysts,  the  exact  determination 
of  the  trachytes  becomes  less  difficult.  Quartz  fails  and  feldspars 
constitute  the  prominent  porphyritic  crystals.  The  greater  num- 
ber of  the  feldspars  should  show  no  striations  when  the  cleavage 
faces  are  examined  with  a  lense.  The  clear  vitreous  variety  of 
orthoclase  which  frequently  appears  in  the  later  volcanic  rocks 
and  especially  in  the  trachytes  was  called  sanidine  by  the  early 
observers  and  the  name  sanidine  is  therefore  often  employed  even 
to-day.  Though  visible,  the  dark  silicates  constitute  but  a  sub- 
ordinate part  of  the  rock. 

As  the  phenocrysts  become  abundant  the  trachytes  proper  pass 
into  the  Trachyte-porphyries.  The  groundmass  is  as  a  rule  felsitic. 
The  cellular  texture  disappears  entirely  and  the  rocks  are  dense 
and  characteristically  porphyritic.  The  interiors  of  thick  surface 
flows,  the  dikes,  intrusive  sheets  and  the  outer  parts  of  laccoliths 
are  their  special  home. 

When  phenocrysts  are  in  marked  excess  over  the  groundmass 
and  constitute  the  greater  part  of  the  rock,  the  Syenite-porphyries 
result.  The  groundmass  is  rather  coarsely  felsitic  and  becomes 
increasingly  coarse  as  the  true  syenites  are  approached.  All  the 
minerals  forming  phenocrysts  are  now  not  difficult  to  recognize. 
The  syenite-porphyries  are  met  in  deep-seated  dikes,  thick  intrusive 
sheets  and  in  the  central  parts  of  laccoliths.  They  mark  a  textural 
transition  to  the  syenites. 

Synonyms  and  Relatives. —  The  name  trachyte  is  an  old  one, 
having  been  first  given  in  1822  by  the  Abbe  Hauy  to  volcanic 


IGNEOUS  ROCKS.  43 

rocks  from  the  Auvergne  in  France,  whose  rough  and  rasping  sur- 
faces suggested  its  creation  from  the  Greek  adjective  meaning 
rough.  For  thirty  years  or  more  it  was  used  for  the  light-colored 
volcanic  rocks  which  are  now  subdivided  among  the  rhyolites, 
trachytes,  dacites  and  andesites ;  and  in  earlier  writers  the  word 
must  often  be  interpreted  in  this  general  sense.  For  many  years 
subsequent  to  1860  and  after  its  mineralogy  became  defined  as 
now,  it  was  restricted  to  the  lavas  of  Tertiary  and  later  age,  while 
"  porphyry  "  was  employed  for  the  corresponding  rocks  of  earlier 
geologic  time.  Porphyry  where  accurately  used  is  now  little  more 
than  a  textural  term,  but  in  common  speech  it  is  applied  loosely  to 
almost  any  eruptive  which  happens  to  be  associated  with  an  ore- 
body  in  the  Cordilleran  region.  When  soda  becomes  especially 
pronounced  in  the  composition  of  a  member  of  the  trachyte  series 
it  leads  to  several  mineralogical  variations  from  the  type.  The 
principal  feldspar  may  become  anorthoclase,  and  then  the  name 
pantellerite  has  been  applied.  Acmite,  the  soda  pyroxene,  may 
appear,  giving  acmite-trachyte.  ^Egirite  may  manifest  itself  as 
may  also  sodalite.  All  of  these  however  cannot  readily  be  identi- 
fied by  the  eye.  They  mark  passages  to  the  phonolites.  Other 
names  of  interest  in  this  connection  are  bostonite,  keratophyre, 
volcanite,  latite,  vulsinite,  trachyandesite,  trachydolerite,  etc.,  all  of 
which  will  be  found  in  the  Glossary. 

Alteration.  —  The  alteration  is  practically  the  same  as  that  de- 
scribed under  rhyolites. 

Distribution.  —  True  volcanic  trachytes  are  extremely  rare  in  this 
country,  for  many  of  the  other  cited  localities,  as,  for  instance, 
some  of  those  in  the  reports  of  the  Fortieth  Parallel  Survey,  have 
been  shown  to  be  andesites.  Beautiful  examples  do,  however,  oc- 
cur in  the  Black  Hills,  with  superbly  developed  orthoclases.  Others 
are  known  in  Custer  County,  Col.  (see  Analysis  i),  and  in  Montana 
(Analyses  4  and  5).  The  trachyte-porphyries,  strictly  so  called, 
are  not  identified  with  certainty  in  very  wide  distribution,  although, 
doubtless,  many  dikes  in  the  West  may  be  properly  described  as 
such.  In  southeast  Missouri,  at  Iron  Mountain  and  Pilot  Knob, 
trachyte-porphyries  are  very  abundant.  Many  interesting  dikes  of 
them  occur  around  Lake  Champlain,  and  among  the  pre-Cambrian 
vblcanics  of  the  Atlantic  Coast  they  are  not  lacking.  Abroad  tra- 


44  A   HAND  BOOK  OF  ROCKS. 

chytes  are  more  common,  and  along  the  Rhine  —  where  the  peak 
of  the  Drachenfels  is  situated,  which  furnishes  the  commonest 
specimens  for  collection  —  in  the  Auvergne,  in  Italy  and  in  the 
Azores  they  are  well  known. 

Trachyte  Tuffs  are  not  common  in  America,  and  offer  only  micro- 
scopic points  of  difference  from  those  formed  of  rhyolitic  material. 

THE  SYENITES. 

SiO,         Al,0,       FeaO,       FeO  CaO        MgO        KiO         Na,O       Loss.     Sp.  Gr. 

1.  60.03        20.76       4.01        0.75         2.62       0.80       5.48        5.96       0.59 

2.  59.83        16.85  7.01          4.43        2.6l        6.57        2.44 


29        2.73 
3.  59.78        16.86        3.08        3.72          2.96        0.69        5.01        5.39  .58        2.689 

4-  59-37  I7-92  6.77  2.02  4.16  1.83  6.68  1.24  0.38      2.71 

5.  56.45  20.08  1.31  4.39  2.14  0.63  7.13  5.61        .77 

6.  46."  14-75  2.20  4.51  7.82  5.73  3.84  1.29        .59      2.904 

7.  46.73  10.05  3-53  8.20  13.22  9.68  3.76  1.81        .24 

I.  Fourche  Mtn.  near  Little  Rock,  Ark.,  J.  F.  Williams  ;  Igneous  Rocks  of  Ark., 
88.  2.  Plauen,  near  Dresden,  F.  Zirkel,  Pogg.  Ann.,  CXXII.,  622.  3.  Custer  Co., 
Colo.  Cross,  Proc.  Colo.  Sci.  Soc.,  1887,  240.  4.  Biella,  Piedmont,  Cossa.,  Turin 
Acad.,  ii.,  XVIII.,  28.  5.  Sodalite-syenite,  Highwood  Mtns.,  Mont.,  W.  Lindgren, 
A.  J.  S.,  Apr.,  1893,  296.  6.  Minette,  Rhode  Island,  badly  decomposed,  contained 
COa  7.32,  Pirsson,  A.  J.  S.,  Nov.,  93,  375.  7.  Shonkinite,  Highwood  Mtns.,  Mont., 
Weed  and  Pirsson,  Bull.  Geol.  Soc.  Amer.,  VI.,  414. 

Comments  on  the  Analyses.  —  The  syenites  mark  a  decrease  in 
SiO2  from  the  granites  and  a  general  increase  in  all  the  bases.  The 
high  percentage  of  alkalies  is  especially  worthy  of  remark,  and  the 
notably  large  amounts  of  soda,  showing  the  passage  to  the  nephe- 
line  syenites.  The  parallelism  with  the  trachytes  is  close.  The 
last  two  analyses  exhibit  excessively  basic  extremes,  whose  theo- 
retical significance  is  commented  on  in  the  next  paragraph. 

Figs.  14  and  15  are  based  respectively  upon  the  average  molec- 
ular ratios  and  the  average  percentages  of  the  above  analyses  ex- 
cept Nos.  6  and  7.  They  show  interesting  contrasts  with  Figs.  10 
and  1 1  of  the  granites,  being  shortened  right  and  left,  and  length- 
ened above  and  below.  The  figures  are  almost  the  same  as  those 
of  the  trachytes. 

Mineralogical  Composition,  Varieties.  —  The  name  syenite  was 
suggested  by  Syene,  now  Assuan,  an  Egyptian  locality,  where  a 
hornblende  granite  was  formerly  obtained  for  obelisks,  and  if  its 
local  significance  were  perpetuated,  syenite  as  formerly  should  be 
applied  to  this  rock.  But  Werner  used  it  in  the  last  century  for 


IGNEOUS  ROCKS. 


45 


the  well-known  typical  rock  from  the  Plauenschen  Grund  (see 
Analysis  2),  near  Dresden,  that  contains  almost  no  quartz,  and  of 
recent  years  this  has  been  its  correct  use.  Typical  syenites  have 
orthoclase  and  hornblende;  those  with  biotite  are  called  mica- 
syenites.  Some  plagioclase  is  always  present  and  magnetite, 
apatite  and  zircon  are  invariable.  When  the  plagioclase  becomes 
equal  in  amount  with  the  orthoclase  the  rocks  are  called  monzonites 
and  they  mark  a  transition  to  the  diorites.  Mica  syenites  in  dikes, 


FIGS.  14  AND  15.  Diagrams  illustrating  the  chemical  composition  of  the  syenites 
whose  analyses  appear  in  the  above  table.  Fig.  14  is  based  on  molecular  ratios  ;  Fig. 
15  on  percentages. 

basic  and  of  dark  color,  have  been  called  minette.  Orthoclase  and 
augite  afford  augite-syenite.  An  excessively  basic  one  (Analysis 
7),  from  the  Highwood  Mountains,  Mont,  has  recently  been  de- 
scribed by  Weed  and  Pirsson  under  the  name  Shonkinite.  It  is  of 
great  theoretical  importance,  as  it  shows  that  orthoclase  is  not  lim- 
ited to  acidic  rocks,  but  may  be  the  prevailing  feldspar  in  very  basic 
ones.  Still  more  recently  J.  P.  Iddings  has  noted  others  of  similar 
character  from  the  region  of  the  Yellowstone  Park.  (Jour,  of 
Geology,  December,  1895,  935.)  Basic  nephelite-syenites  have 
been  earlier  known.  Still  the  table  on  page  23  expresses  the  gen- 
eral truth,  the  exceptions  being  excessively  rare  rocks  so  far  as  yet 
known.  Syenites  are  themselves  rare  rocks.  With  high  soda, 


46  A  HAND  BOOK  OF  ROCKS. 

the  mineral  sodalite  develops  and  yields  sodalite  syenites  which  are 
passage  forms  to  nephelite  syenites. 

Relationships.  —  Syenites  are  most  closely  allied  with  nephelite- 
syenites,  into  which  with  increase  of  soda  they  readily  pass.  They 
also  with  increasing  plagioclase  shade  into  diorites  and  the  augite- 
syenites  are  closely  akin  to  gabbros. 

Geological  Occurrence.  —  Syenites  form  irregular  masses  and 
dikes,  precisely  as  do  granites. 

Alteration.  —  There  is  little  to  be  said  that  was  not  covered  un- 
der granite.  The  rarity  of  syenite  makes  it  a  much  less  serious 
factor.  In  metamorphism  they  pass  into  gneisses. 

Distribution.  —  Syenites  occur  in  the  great  igneous  complex  of 
the  White  Mountains.  They  form  large  knobs  and  dikes  near 
Little  Rock,  Ark.,  and  a  dike  is  known  in  Custer  County,  Colo. 
One  of  the  few  American  minettes  yet  discovered  is  a  dike  on 
Conanicut  Island,  R.  I.,  described  by  Pirsson  (see  Analysis  7). 
Abroad,  syenites  are  better  known.  The  Plauenscher  Grund,  near 
Dresden,  Biella  in  the  Piedmont,  and  the  vicinity  of  Christiania, 
Norway,  are  the  best  known.  Minettes  are  especially  famous  in 
connection  with  the  mining  district  about  Freiberg,  Saxony,  and  in 
the  Vosges  mountains. 

THE   PHONOLITE-NEPHELITE-SYENITE  SERIES. 
THE  PHONOLITES. 

SiO,  Al.O,  Fe,O,  FcO  CaO  MgO  K,O  Na,O  Lost.  Sp.  Gr. 

1.  61.08  18.71  1.91  0.63  1.58  0.08  4.63  8.68  2.21  2.582 

2.  60.02  20.98  2.21  0.51  1.18  tr.  5.72  8.83  0.70  2.576 

3.  59.46  23.00  3.52  ...  i.  oo  0.50  4.90  7.13  0.71 

4-  59-1?  19-74  3-39       —  0.92  0.15  6.45  8.88  1.18       2.566 

5.  56.43  22.25  2*66  °-97  I-4I        tr.  2.77  11.12  2.05        2.54 

6.  49. 18  20.65  —  5-97  2.43  0.29  6.88  9.72  1.60       2.553 

7.  45.18  23.31  6.ii        ...  4.62  1.45  5.94  11.17  1.14 

8.  44.50  22.96  6.84        ...  8.65  1.65  4.83  6.70  2.06 

I.  Mato  Tepee  or  Devil's  Tower,  near  Black  Hills,  Wyo.,  Pirsson,  A.  J.  S.,  May, 
1894,  344.  2.  El  Paso  Co.,  Colo.,  Cross,  Proc.  Col.  Sci.  Soc.,  1887,  169.  3.  Island 
of  Fernando  de  Noronha,  Brazil,  Gttmbel,  Tscher.  Mitt.,  1880,  II.,  188.  4.  Near 
Zittau,  Saxony,  v.  Rath,  Z.  d.  d.  g.  G.,  VIII.,  297.  5.  Wolf  Rock,  Cornwall,  Eng., 
Phillips,  Geol.  Mag.,  VIII. ,  249.  6.  Leucite-phonolite,  near  Rieden,  Germany,  Zirkel, 
Lehrbuch,  II.,  465.  7.  Eleolite-porphyry,  Beemerville,  N.  J.,  J.  F.  Kemp,  N.  Y.  Acad. 
Sci.,  XI.,  69.  8.  Eleolite-porphyry,  Magnet  Cove,  Ark.,  J.  F.  Williams,  Igneous 
Rocks  of  Ark.,  261. 


IGNEOUS  ROCKS. 


47 


Comments  on  the  Analyses.  —  It  is  at  once  apparent  from  the 
analyses  that  the  range  in  silica,  except  in  the  last  two,  is  much 
like  that  of  the  trachytes,  but  that  the  alumina  goes  higher,  and 
that  the  alkalies  are  in  extremely  large  amounts.  No  other  rocks, 
except  the  corresponding  granitoid  types,  reach  these  amounts  in 
alkalies.  The  soda  which  is  necessary  for  the  formation  of  the 
nepheline  is  naturally  in  excess.  The  rare  leucite-phonolites,  as  a 
general  thing,  are  more  basic  and  show  comparatively  high  potash. 
The  last  two  analyses  of  intrusive  or  dike  members  are  abnormally 
basic  for  phonolitic  rocks. 


FIGS.  1 6  AND  17.  Diagram  illustrating  the  chemical  composition  of  the  phonolites 
whose  analyses  appear  in  the  above  table.  Fig.  16  is  based  on  molecular  ratios  ;  Fig. 
1 7  on  percentages. 

Figs.  1 6  and  17  are  based  respectively  upon  the  average  molec- 
ular ratios  and  the  average  percentages  of  the  first  five  analyses  in 
the  above  table.  In  analyses  3  and  4  it  was  however  necessary  to 
make  an  adjustment  of  the  percentage  of  Fe2O3  with  the  FeO 
which  had  not  been  determined  separately.  The  amounts  are, 
however,  in  any  event,  so  small  as  not  appreciably  to  affect  the 
diagrams.  The  pronounced  development  of  the  alkalies  and 
alumina  below  the  horizontal  line  comes  out  forcibly  and  furnishes, 
interesting  contrasts  with  the  rhyolites. 


48  A  HAND  BOOK  OF  ROCKS. 

General  Description.  —  The  Phonolite  Series  embraces  a  group 
of  rocks  not  often  easy  of  identification  without  the  microscope. 
They  are  rare  and  are  seldom  met  by  the  field  geologist  or  engi- 
neer. When  they  are  found,  however,  they  afford  exceptionally 
interesting  material  for  detailed  study,  and,  inasmuch  as  they 
have  been  discovered  in  more  recent  years  in  association  with 
some  of  our  most  productive  gold  deposits,  they  possess  an  impor- 
tance for  the  mining  engineer  which  they  formerly  lacked.  The 
rocks  of  the  phonolite  series  are  usually  dense  and  finely  crystal- 
line ;  they  are  very  seldom  vesicular  or  even  glassy.  Dull  green 
and  gray  are  the  common  colors,  but  as  they  approach  the 
trachytes  they  become  lighter  in  shade.  The  light-colored  min- 
erals are  in  excess,  orthoclase  being  the  most  important  single 
component  and  the  only  one  which  is  usually  large  enough  to  be 
recognized  by  the  eye  alone.  The  nephelite  is  almost  always  too 
small  to  be  visible  without  the  microscope.  Its  easy  gelatinization, 
however,  makes  it  possible  for  the  observer  often  to  detect  it  by 
simple  chemical  tests.  Thus  a  small  sample  of  the  rock  in  ques- 
tion is  finely  powdered  and  gently  warmed  in  very  dilute  nitric 
acid.  The  nephelite  passes  readily  into  i  lution  and  when  the 
liquid  is  decanted  from  the  undissolved  grains  and  is  boiled  down 
well  toward  dryness,  gelatinous  silica  results.  No  other,  common, 
rock-making  and  gelatinizing  mineral  is  so  easily  soluble  as  nephe- 
lite, olivine  alone  approaching  it.  The  commonest  dark  silicate  in 
the  phonolites  is  augite  and  its  little  dark  glistening  prisms  may 
occasionally  be  recognized.  Hornblende  is  very  rare,  and  biotite 
is  almost  never  seen.  All  these  minerals  are  only  visible  when 
present  as  phenocrysts  ;  the  components  of  the  groundmass  cannot 
be  resolved  by  the  eye  alone.  The  fusing  point  of  the  phonolites 
is  less  than  that  of  the  trachytes,  being  somewhat  under  2000°  F. 
(1090°  C). 

The  Phonolites  proper  are  felsitic  or  slightly  porphyritic  rocks, 
which  are  not  always  easily  to  be  distinguished  from  felsitic  varie- 
ties of  trachytes  and  andesites.  They  are,  however,  characteristic- 
ally dense,  and  as  the  rocks  often  have  a  peculiar  and  marked  ten- 
dency to  break  up  into  thin  slabs  or  plates,  which  ring  musically 
under  the  hammer,  they  sometimes  reveal  themselves  in  this  way. 
Fig.  1 8  reproduces  a  very  striking  outcrop  of  phonolite  which 


FIG.  18.     View  of  an  exposure  of  platy  phonolite,  Sugar  Loaf  Mountain,  Black  Hills, 
S.  D.     J.  D.  Irving,  Annals  N.  Y.  Acad.  Sci.,  XIV.,  PI.  IX.,  1899. 


IGNEOUS  ROCKS.  49 

shows  this  property.  The  chemical  test  mentioned  above  should 
always  be  used  in  corroboration  before  the  identification  is  positively 
made.  The  phonolites  proper  are  found  in  surface  flows  and 
dikes. 

The  Phonolite-porphyries  result  when  the  phenocrysts  become 
notably  abundant.  The  phenocrysts  are  then  chiefly  orthoclase 
with  a  few  augites,  and  perhaps  with  an  occasional  titanite. 
Nephelite  in  porphyritic  crystals  is  known  from  a  few  localities  but 
is  seldom  seen.  The  phonolite-porphyries  occur  in  dikes  and  in- 
truded sheets.  When  the  phenocrysts  constitute  the  greater  part 
of  the  rock  the  Nephelite-syenite  porphyries  are  developed.  They 
are  extremely  rare  rocks  and  mark  the  passage  to  the  nephelite 
syenites. 

Synonyms  and  Relatives.  —  The  name  phonolite  is  an  old  one. 
It  was  given  by  Klaproth  in  1801  to  the  rocks  which  had  long 
been  called  clinkstone  and  was  merely  the  Greek  equivalent  of  this 
colloquial  term.  Phonolite  was  formerly  restricted  to  Tertiary  and 
later  eruptives  but  no  time  distinction  is  longer  implied  when  it  is 
used,  although  as  a  matter  of  fact  most  of  the  known  phonolites 
belong  to  this  portion  of  geological  time.  The  phonolites  are 
closely  related  to  the  trachytes,  but  they  have  more  soda  and 
alumina  and  at  the  same  time  not  enough  silica  to  form  albite. 
Thus  as  the  silica  rises  there  comes  a  time  when  albite  can  absorb 
all  the  soda  and  then  nephelite  becomes  an  impossibility.  All  the 
more  can  nephelite  never  appear  with  original  quartz,  because 
quartz  itself  is  an  impossibility  until  all  the  albite  possible  has  been 
produced.  The  abundant  soda  in  the  phonolite  magma  occasions 
the  frequent  production  of  noselite  and  hauynite,  but  they  can  seldom 
be  detected  with  the  eye  alone.  For  the  same  reason  the  dark 
green,  acicular,  soda  pyroxene  aegirite  frequently  takes  the  place  of 
the  augite  and  in  the  groundmass  may  constitute  a  perfect  felt  of 
little  needles.  This  variety  of  phonolite  is  called  tinguaite,  but  it 
also  cannot  be  readily  determined  by  the  unassisted  eye.  Apachite, 
gieseckite-porphyry,  liebnerite-porphyry,  and  sussexite  are  rocks 
related  to  the  phonolites  and  will  be  found  defined  in  the  Glossary. 

With  the  increase  of  orthoclase  and  the  decrease  of  nephelite 
the  phonolite  series  passes  into  the  trachytes  with  which  they  are 
in  all  respects  closely  akin.  With  the  increase  of  plagioclase  and 
4 


50  A  HAND  BOOK  OF  ROCKS. 

the  dark  silicates  they  pass  in  the  opposite  direction  into  certain 
basaltic  rocks  with  nephelite. 

The  leucite  rocks  of  trachytic  affinities  constitute  a  rare  and 
minor  group  of  the  phonolite  series  from  which  they  might  with 
propriety  be  separated  to  form  a  series  of  their  own.  They  are, 
however,  so  rare  that  they  are  only  mentioned  here  under  the 
phonolites.  When  the  potash  in  the  magma  becomes  relatively 
rich  and  the  silica  so  poor  that  there  is  more  than  enough  of  the 
former  and  too  little  of  the  latter  to  yield  orthoclase,  leucite  be- 
comes a  possibility.  Hence  it  follows  that  leucite  and  orthoclase 
usually  go  together  and  that  leucite  is  sometimes  found  with 
nephelite,  but  as  soon  as  the  silica  becomes  abundant  enough  to 
combine  with  all  the  potash  and  its  attendant  alumina,  to  yield 
orthoclase,  leucite  is  an  impossibility.  All  the  more  do  we  thus 
never  find  leucite  with  original  quartz.  Felsitic  or  porphyritic 
rocks  with  leucite,  orthoclase  and  augite  or  some  related  dark 
silicate  are  usually  called  leucite  trachyte.  If  to  this  aggregate 
nephelite  be  added  leucite-phonolite  results.  The  related  rocks 
leucitophyre,  orendite  and  wyomingite  will  be  found  defined  in  the 
Glossary. 

Alterations.  —  The  nephelite  changes  quite  readily  to  natrolite 
and  perhaps  analcite,  while  leucite  yields  analcite.  Metamorphic 
processes  are  yet  to  be  studied. 

Distribution.  —  The  true  volcanic  phonolites  are  only  known  in 
a  few  localities  in  this  country,  such  as  the  Black  Hills,  where  they 
form  dikes,  sheets  and  isolated  buttes  (Devil's  Tower),  and  the 
Cripple  Creek  mining  district  of  Colorado,  where  the  comparatively 
few  dikes  known  have  proved  of  great  importance  as  associates  of  the 
ores.  Nephelite-  or  eleolite-porphyries  (tinguaites)  are  exceedingly 
rare  rocks  and  have  been  found  near  Magnet  Cove,  Ark.,  and  Beem- 
erville,  N.  J.,  associated  with  nephelite-syenite.  Phonolites  are 
much  more  abundant  abroad,  being  well  known  in  many  parts  of 
Germany.  The  varieties  with  leucite  are  especially  familiar  from 
the  vicinity  of  Rieden,  in  the  extinct  volcanic  district  of  the  Eifel. 
A  peculiar  leucite  rock,  with  abundant  scales  of  phlogopite,  gives 
the  name  to  the  Leucite  Hills,  two  or  three  miles  north  of  Point  of 
Rocks,  Wyo.  Leucite  tinguaites  occur  near  Magnet  Cove,  Ark., 
in  the  Highwood  Mountains,  Mont.,  and  near  Rio  Janeiro,  Brazil. 


IGNEOUS  ROCKS.  51 

Tuffs  are  known  abroad  but  not  in  this  country,  and  exhibit  few 
features  calling  for  special  mention. 

THE  NEPHELITE  SYENITES. 


I. 

2. 

3- 
4- 
5- 
6. 

7- 
8. 

9- 

10. 

SiO, 
60.39 
59-70 
59-Qi 
56.30 
54.20 

52-75 
51.90 
50.96 
50-36 
41-37 

A1S0, 
22.51 
18.85 

18.18 
24.14 
21.74 
22.55 
22.54 
19.67 
19.34 
16.25 

Fe,0, 
0.42 
4.85 
1.63 
1.99 
0.46 
3-65 
4-03 
7.76 
6-94 
16.93 

FeO 
2.26 

3-65 
2.36 
3-15 

CaO 
0.32 

1-34 
2.40 
0.69 
1-95 
1.85 
3-" 
4.38 
3.43 
"•35 

MgO 

0.13 

0.68 
1.05 
0.13 
0.52 
0.15 
1.97 
0.36 

4-57 

K,0 

4-77 
5-97 
5-34 
6.79 
6.97 
7-05 
4.72 
6.77 
7.17 
3.98 

Nm,O 
8.44 
6.29 
7-03 
9.28 
8.69 
8.10 
8.18 
7.67 
7.64 
4.18 

Loss. 
0.57 
1.88 
0.50 

1.58 
3.60 

0.22 
1.38 
3.51 

0-45 

Sp.Gr. 

I.  So-called  Nephelite-syenite,  or  Litchfieldite,  Litchfield,  Me.,  W.  S.  Bayley,  G. 
S.  A.,  III.,  241.  2.  Nephelite-syenite,  Fourche  Mountains,  Ark.,  J.  F.  Williams, 
Igneous  Rocks  of  Ark.,  88.  3.  Nephelite-syenite,  Red  Mountain,  N.  H.,  W.  S. 
Bayley,  G.  S.  A.,  III.,  250.  4.  Ditroite,  Hungary,  Fellner,  Neues  Jahrb.,  1868,  83. 
5.  Foyaite,  Portugal,  Jannasch,  Neues  Jahrb.,  II.,  II.  6.  Nephelite-syenite,  Sao 
Paulo,  Brazil,  Machado,  Tsch.  Mitt.,  IX.,  1888,  334.  7.  Laurdalite,  variety  of 
Nephelite-syenite.  Lund,  Norway,  Br5gger,  Syenit-pegmatit-gange,  33.  8.  Leucite- 
syenite,  Arkansas,  J.  F.  Williams.  Igneous  Rocks  of  Ark.,  276.  9.  Nephelite-syenite, 
Beemerville,  N.  J.,  F.  W.  Love  for  J.  F.  K.t  N.  Y.  Acad.  Sci.,  XI.,  66.  10.  Basic 
Nephelite-syenite,  Beemerville,  N.  J.,  J.  F.  Kemp.,  N.  Y.  Acad.  Sci.,  XI.,  86. 

Comments  on  the  Analyses.  — A  considerable  range  is  shown  in 
the  SiO2,  some  analyses  going  below  the  usual  percentages  for 
syenites  and  the  last  analysis  being  abnormal.  In  general  the 
amounts  of  alkalies  are  extremely  high,  with  Na2O  in  excess,  in 
which  respect  the  phonolites  are  paralleled. 

Figs.  19  and  20  are  based  respectively  upon  the  average  molec- 
ular ratios  and  the  average  percentages  of  the  first  seven  analyses 
in  the  above  table.  In  Nos.  2,  4,  and  6  it  has  been  necessary  to 
adjust  the  undetermined  percentages  of  FeO,  but  the  error,  if  one 
is  introduced,  is  not  great  in  any  event.  The  figures  closely  re- 
semble those  of  the  phonolites  and  present  the  same  general  pecu- 
liarities. 

Mineralogical  Composition  and  Varieties.  —  The  minerals  of neph- 
elite-syenite  are  in  general  the  same  as  those  of  syenite  proper, 
with  the  addition  of  nephelite,  often  sodalite,  and  several  charac- 
teristic ones  into  which  the  rare  earths  enter  as  bases.  Zircon 
is  widespread  and  is  often  large  enough  to  afford  fine  crystals. 


52  A   HAND  BOOK  OF  ROCKS. 

For  this  reason  the  rocks  were  named  zircon-syenite  many  years 
ago.  The  nephelite  is  often  called  eleolite  (or  elaeolite),  from  the 
former  custom  of  speaking  of  this  mineral  in  pre-Tertiary  rocks  as 
eleolite  and  in  later  ones  as  nephelite,  just  as  we  have  had  ortho- 
clase  and  sanidine,  but  the  custom  is  gradually  falling  into  disuse. 
Attempts  have  been  made  to  give  different  names  according  to  the 
dark  silicate ;  for  instance,  those  with  hornblende  were  called  foyaite, 
from  Foya,  a  Portuguese  locality  ;  those  with  biotite,  miascite  from 
Miask,  in  the  Urals.  But  both  these  minerals  so  often  appear  to- 
gether or  with  pyroxene  that  the  practice  is  not  generally  observed, 
Ditroite  is  a  variety  rich  in  blue  sodalite.  The  Litchfield,  Maine, 


FIGS.  19  AND  20.  Diagrams  illustrating  the  chemical  composition  of  the  nephelite- 
syenites  which  appear  in  the  above  table.  Fig.  19  is  based  on  molecular  ratios  ;  Fig. 
20  on  percentages. 

rock  has  been  shown  by  Bayley  to  have  as  its  feldspar  albite  al- 
most exclusively,  and  he,  therefore,  has  called  it  litchfieldite.  The 
texture  of  nephelite-syenites  varies  very  much.  At  times  it  is  very 
coarsely  granitoid,  and  again  it  is  what  is  called  trachytic,  i.  e.,  with 
little  rods  of  feldspar,  more  or  less  in  flow  lines,  like  a  trachyte  and 
marking  a  passage  to  the  phonolites.  Types  have  been  based  on 
these  characters.  Where  at  all  finely  crystalline,  the  determination 
of  nephelite-syenites,  as  against  true  syenites,  is  a  matter  for  the 


IGNEOUS  ROCKS.  53 

microscope.  Nephelite-syenites  are  comparatively  rare  rocks. 
Corresponding  rocks  with  leucite  are  as  yet  only  known  from 
Arkansas  and  Montana. 

Relationships.  —  As  already  remarked,  the  nephelite-syenites  are 
closely  related  to  the  true  syenites,  and  to  the  phonolites.  With 
certain  basic  plagioclase  rocks  with  nephelite,  called  theralites,  they 
are  also  of  near  kinship. 

Geological  Occurrence,  Alteration.  —  The  nephelite-syenites  are 
specially  prone  to  appear  as  dikes,  often  on  a  very  large  scale. 
Their  alteration  affords  no  special  features,  as  distinguished  from 
the  syenites  or  granites,  except  as  regards  the  secondary  minerals 
from  the  nephelite.  Natrolite,  muscovite  and  kaolin  are  all  known 
in  this  relation  and  the  last  two  have  been  called  liebenerite  and 
gieseckite.  Cancrinite  also  results  from  the  alteration  of  nephelite. 
The  rarity  of  the  nephelite-syenites  has  prevented  their  playing  an 
important  role  among  metamorphosed  rocks. 

Distribution.  —  Nephelite-syenites  are  known  in  North  America 
at  Montreal  and  Dungannon,  Ont ;  Litchfield,  Me. ;  Red  Hill,  N. 
H.  ;  Salem,  Mass.  ;  Beemerville,  N.  J.,  where  a  superb  dike  is 
exposed  ;  near  Little  Rock,  Ark.,  where  the  area  is  extensive ;  in 
the  San  Carlos  Mountains,  Tamaulipas,  Mexico,  and  at  several  less 
well  known  localities.  Very  interesting  ones  occur  near  Ria 
Janeiro,  and  in  the  State  of  Sao  Paulo,  Brazil.  Abroad  the  Portu- 
guese locality,  in  the  Monchique  Mountains ;  the  one  at  Ditro,  in 
Hungary,  and  the  wonderful  dikes  near  Christiania,  in  Norway,  so 
prolific  in  rare  minerals,  are  of  especial  interest. 


FIG.  21.     Diagrams  illustrating  the  chemical  composition  of  Wyomingite,  a  leucite 
rock.     The  upper  is  based  on  molecular  ratios ;  the  lower  on  percentages. 


54  A  HAND  BOOK  OF  ROCKS. 

The  Trachyte-Nephelite-Syenite  Table.  —  The  same  generalities 
apply  regarding  the  preparation  of  this  table  as  are  given  on  p. 
38.  The  recasting  brings  out  several  features  which  might  not 
otherwise  be  apparent.  Thus  in  the  first  rock  used  there  is  some 
quartz,  but  it  was  not  considered  sufficient  by  the  original  observer 
to  place  the  rock  in  the  rhyolites.  Among  the  feldspars,  ortho- 
clase  is  sometimes  in  excess,  sometimes  albite.  Together  they 
constitute  from  one  half  to  three  quarters  of  the  mass.  The  excep- 
tion to  this  relation  is  the  unusual  rock  shonkinite  which  is  given 
under  44.7,  which  is  so  low  in  silica  as  to  have  almost  closer 
affinities  with  the  basic  rocks,  such  as  gabbros,  than  with  the 
syenites.  Anorthite  is  low  in  all  these  rocks.  The  exigencies  of 
recasting  make  the  use  of  the  nephelite  molecule  unavoidable. 
With  high  Na2O  and  low  SiO2  it  becomes  necessary  to  infer  this 
relatively  basic  feldspathoid,  even  in  some  trachytes  and  syenites. 
As  given  in  the  table  it  may  represent  some  sodalite.  Where 
hydration  is  marked  kaolinite  must  be  assumed.  The  totals  of 
the  light-colored  minerals  are  high.  Aside  from  shonkinite,  they 
range  from  a  maximum  of  97.5  to  a  minimum  of  86.5,  implying 
thus  a  variation  of  the  dark  silicates  from  a  minimum  of  2.5  to  a 
maximum  of  13.5,  volumes  in  every  case  being  used.  Under 
shonkinite  however  the  ratios  are  changed,  there  being  practically 
40  per  cent,  light-colored  as  against  60  per  cent.  dark.  Amphi- 
boles  or  pyroxenes  are  the  chief  dark  minerals,  olivine  only  appear- 
ing in  the  shonkinite.  The  proportions  of  nephelite  under  the 
phonolites  and  nephelite-syenites  are  of  interest  in  that  they  show 
the  general  amounts  of  this  mineral.  We  often  cannot  detect  the 
nephelite  without  the  microscope. 


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CHAPTER   IV. 

THE  IGNEOUS  ROCKS,  CONTINUED.    THE  DACITE-QUARTZ-DIORITE 
SERIES  AND  THE  ANDESITE-DIORITE  SERIES. 

THE   DACITE-QUARTZ-DIORITE  SERIES. 
THE  DACITES. 

Sp.  Gr. 


SiO, 

Al,0, 

Fe.O, 

FeO 

C*0        1 

IgO 

Na,O 

K,0 

Loss. 

s. 

69.96 

15-79 

2.50 

1-73        < 

3.64 

3.80 

4.12 

i-53 

a. 

69.36 

16.23 

0.88 

1-53 

3-17 

•34 

4.06 

3.02 

0.45 

3- 

67.49 

16.18 

1.30 

1.22 

2.68 

•34 

4-37 

2.40 

2.69 

4- 

67.2 

17.0 

3-5 

1.2 

4-5 

•  5 

3-7 

1.6 

0.9 

5- 

67.03 

16.27 

3-97 

3-42 

•19 

2.71 

3-5° 

1.56 

6. 

66.03 

14-57 

2-57 

I.I9 

3.38 

.89 

3-71 

2.70 

2.07 

7- 

63.36 

16.35 

2.12 

3-05 

4-79       . 

J.28 

3.58 

2.92 

0.99 

I.  McClelland  Peak,  near  Comstock  Lode,  Nev.,  F.  A.  Gooch,  Bull.  17,  U.  S.  G. 
S.,  33.  2.  Lassen's  Peak,  California,  Hague  and  Iddings,  A.  J.  S.,  Sept.,  1883,  232. 

3.  Sepulchre  Mountain,  Yellowstone  Park,  J.  P.  Iddings,  Phil.  Soc.  Wash.,  XI.,  210. 

4.  Nagy-Sebes,  Hungary,  Doelter,  Tscher.  Min.  Petr.  Mitt.,  1873,  93.     5.  Eureka 
Dist.,  Nev.    A.  Hague,  Mono.  XX.,  U.  S.  G.  S.,  264.     6-7.  Colombia,  S.  America, 
From  K.flch's  Petrographie  of  Colombian  Volcanoes,  quoted  in  Jour.  Geol.,  I.,  171. 

Comments  on  the  Analyses.  —  It  appears  at  once  from  the  analyses 
that  the  dacites  are  high  in  silica,  in  which  they  equal  the  lower 
ranges  of  rhyolites.  As  compared  with  the  latter,  soda  is  prevail- 
ingly in  excess  of  potash,  and  as  a  rule  the  other  bases  run  higher 
and  especially  the  lime. 

The  diagrams  in  Figs.  22  and  23,  show  considerable  similarity 
with  those  of  the  rhyolites,  but  on  close  comparison  it  will  appear 
that  in  the  former,  the  soda  is  in  marked  excess  over  the  potash 
and  both  the  lime  and  the  magnesia  are  represented  by  longer 
intercepts. 

General  Description.  —  The  Dacites  embrace  a  group  of  rocks 
which  so  strongly  resemble  the  rhyolites  as  often  to  make  it  dif- 
ficult, if  not  impossible,  for  an  observer  to  positively  identify  them 
as  against  the  latter.  The  light-colored  minerals  are  the  ones 
which  give  character  to  the  group.  Quartz  and  feldspar  are 
the  prominent  components  and  the  prevailing  feldspar  is  plagioclase, 
and  one  of  the  more  acidic  varieties.  Biotite  is  perhaps  the  most 

56 


IGNEOUS  ROCKS. 


57 


common  of  the  dark  silicates  but  both  hornblende  and  augite  are 
frequent.  The  minor  accessories,  apatite,  zircon,  magnetite,  etc., 
are  seldom  visible  to  the  eye.  The  prevailing  colors  are  light 


s;o 


FIGS.  22  AND  23.  Diagrams  illustrating  the  chemical  composition  of  the  dacites 
which  are  given  in  the  above  analyses.  Fig.  22  is  based  on  molecular  ratios  ;  Fig.  23 
on  percentages. 

grays,  yellows  and  pale  reds.  The  fusing  point  is  perhaps  slightly 
less  than  that  of  the  rhyolites.  Glasses  and  cellular  textures  are 
not  uncommon. 

The  textures  of  the  dacites  range  from  felsitic  to  coarsely  por- 
phyritic.  The  Dacites  proper  are  felsitic  to  moderately  porphyritic 
rocks,  sometimes  cellular  from  their  crystallization  as  surface  flows. 
When  finely  felsitic  their  components  cannot  be  distinguished  and 
recognized  with  the  eye  alone,  and  then  the  microscope  is  the  sole 
resource  for  accurate  determination.  They  can  otherwise  only  be 
called  felsites.  When,  however,  the  phenocrysts  become  prominent 
the  only  possible  question  is  between  dacites  and  rhyolites,  for  these 
are  the  only  two  with  quartz  in  this  relation.  The  observer  must 
then  study  the  cleavage  faces  of  the  feldspars  with  a  good  lense, 
and  if  the  greater  number  of  these  display  the  striations  peculiar  to 
plagioclase  the  identification  of  the  dacites  can  be  satisfactorily  made. 

When  the  phenocrysts  become  abundant  the  dacites  proper  pass 
into  the  Dacite-porphyries.  The  groundmass  is,  as  a  rule,  felsitic. 
The  cellular  texture  disappears  entirely  and  the  rocks  become 
dense  and  characteristically  porphyritic  types.  The  interiors  of 


58  A  HAND  BOOK  OF  ROCKS. 

thick  surface  flows,  the  dikes,  intrusive  sheets  and  the  outer  parts 
of  laccoliths  are  their  special  homes. 

When  phenocrysts  are  in  marked  excess  over  the  groundmass 
and  constitute  the  greater  part  of  the  rock,  the  Quartz-diorite 
porphyries  result.  The  groundmass  is  rather  coarsely  felsitic  and 
becomes  increasingly  coarse  as  the  quartz  diorites  are  approached. 
All  the  minerals  forming  the  phenocrysts  are  now  not  difficult  to 
recognize.  The  quartz-diorite  porphyries  are  met  in  deep-seated 
dikes,  thick  intrusive  sheets,  and  in  the  central  parts  of  laccoliths. 
They  mark  a  textural  transition  to  the  quartz  diorites. 

Synonyms  and  Relatives.  —  The  name  dacite  was  created  in  1 863 
by  an  Austrian  geologist,  G.  Stache,  who  had  been  working  upon 
the  eruptives  of  the  old  Roman  province  of  Dacia,  now  in  the  dis- 
trict of  Hungary  known  as  the  Siebenbiirgen.  Under  it  was  em- 
braced a  series  of  rocks  somewhat  indefinitely  called  by  earlier 
lithologists  andesitic  quartz-trachytes,  and  other  undesirable 
names.  The  name  dacite  has  proved  to  be  a  useful  one  and  is 
quite  universally  employed  to-day.  The  dacites  were  originally 
considered  to  be  necessarily  Tertiary  or  later  in  geological  age  but 
now  no  time  restriction  is  applied  to  them.  Varieties  are  some- 
times made  on  the  basis  of  the  dark  silicate  present  such  as  mica- 
dacite,  hornblende-dacite  or  augite-dacite.  The  dacites  are  close 
relatives  of  the  andesites  into  which  they  pass  with  increasing 
basicity,  and  with  the  disappearance  of  quartz.  They  are  also 
very  closely  akin  to  the  rhyolites  and  to  those  passage  rocks  from 
rhyolites  to  dacites,  called  pantellerites  and  keratophyres,  which 
are  defined  in  the  Glossary.  Quartz-porphyrite  is  an  old  synonym 
of  dacite  porphyry. 

Alteration,  Metamorphism.  —  The  alteration  of  the  dacites  is 
practically  like  that  of  the  rhyolites,  but  the  greater  abundance  of 
soda-lime  feldspar  may  yield  a  trifle  more  calcite.  The  light- 
colored  silicates  change  to  kaolin.  In  metamorphism  the  dacites 
yield  siliceous  schists  especially  when  greatly  mashed  or  sheared. 

Tuffs.  —  The  tuffs  and  breccias  are  essentially  like  those  of  the 
rhyolites.  From  them  on  account  of  the  almost  universal  advance 
of  alteration  they  cannot  readily  be  distinguished  without  the 
microscope  and  even  then  the  sharp  determination  may  present 
great  difficulties. 


IGNEOUS  ROCKS.  59 

Distribution.  —  Dacites  usually  appear  as  subordinate  members 
in  eruptive  regions  where  the  andesites  are  the  chief  rocks.  They 
are  therefore  widespread  in  the  volcanic  districts  of  the  Cordilleran 
region  and  of  Central  and  South  America. 

THE  QUARTZ-DIORITES. 

SiO,        A1,O,       Fc,O,        FeO         CaO        MgO         N»,O         K,O        LOM. 

1.  70.36       15.47       0.98       1.17       3.18       0.87        4.91         1.71        1.06 

2.  67.54  17.02  2.97  0.34  2.94  1.51  4.62  2.28  0-55 

3.  65.27  15.76  1.36  3.44  3.70  2.14  4.57  3.97  0.42 

4.  63.97  15.78  2.35  1.87  3.71  2.84  4.36  4.01  0.58 

5.  62.43  17.88  1.78  3.53  3.43  4.50  3.10  2.75  1.37 

I.  Quartz-diorite,  Enterprise,  Butte  Co.,  Calif.,  H.  W.  Turner,  Anals.  by  W.  F. 
Hillebrand,  14  Ann.  Rep.  U.  S.  Geol.  Survey,  482,  1894.  2.  Quartz-mica-diorite, 
Electric  Peak,  Yellowstone  Park,  J.  P.  Iddings,  Anal,  by  Whitefield,  Bull.  Phil.  Soc. 
of  Washington,  II.,  206.  3.  Quartz-augite-diorite,  Watab,  Minn.,  A.  Streng,  Neues 
Jahrbuch,  1877,  232.  4.  Quartz-mica-diorite,  Crandall  Basin,  Wyo.,  J.  P.  Iddings, 
Mono.  32,  U.  S.  G.  S.,  p.  261,  W.  H.  Melville,  Analyst.  5.  Quartz-mica-diorite, 
Omeo,  Viet.,  A.  W.  Howitt,  Trans.  Roy.  Soc.  Viet,  XXII.,  99. 

Comments  on  the  Analyses. — The  quartz-diorites,  although 
acidic  rocks,  do  not  have  as  high  percentages  of  silica  as  the 
granites,  but  in  lime  and  soda  they  range  slightly  higher  on  ac- 
count of  the  prevailing  plagioclase.  In  general  they  strongly 
resemble  the  granites  and  diagrams  based  on  the  above  analyses 
would  hardly  differ  from  those  of  the  granites  given  earlier. 

Mineralogical  Composition ,  Varieties.  —  The  quartz-diorites  are 
granitoid  rocks  whose  chief  feldspar  is  plagioclase  and  which  con- 
tain also  quartz  as  an  essential  component.  The  dark  silicates  are 
hornblende  and  biotite,  one  or  both.  The  light-colored  minerals 
are  in  excess  over  the  dark  ones,  but  this  relationship  is  less  pro- 
nounced in  the  more  basic  varieties.  The  typical  mineralogical 
aggregate  contains  hornblende.  When  biotite  is  the  chief,  dark 
silicate  the  rocks  are  called  quartz-mica-diorites.  The  fusing 
point  of  the  rocks  is  a  shade  less  than  that  of  the  granites. 

From  all  other  rocks  except  granites  the  quartz-diorites  are 
distinguished  by  their  granitoid  texture  and  quartz.  From  the 
granites  the  prevalence  of  striated  feldspar  is  the  chief  distinction. 

Relationships. — The  quartz-diorites  are  close  relatives  of  the 
granites  on  the  one  hand  and  of  the  diorites  on  the  other.  To 
the  former  group  an  easy  transition  is  afforded  by  the  grano- 


60  A  HAND  BOOK  OF  ROCKS. 

diorites,  while  the  so-called  quartz-monzonites  mark  a  transition  to 
the  syenites.  Tonalite  and  adamellite  will  be  found  defined  in  the 
Glossary. 

Geological  Occurrence.  —  The  quartz-diorites  form  batholiths, 
dikes  and  local  developments  of  diorites. 

Alteration.  —  The  alteration  is  in  all  essentials  similar  to  that 
of  granite. 

Distribution.  —  Quartz-diorites  occasionally  appear  in  the  eastern 
areas  of  crystalline  rocks.  A  famous  one  with  mica  is  an  important 
member  of  the  Cortlandt  series  of  eruptives  near  Peekskill,  N.  Y. 
Others  with  hornblende  are  known  in  the  Yellowstone  Park  and 
in  the  Sierras. 

THE  ANDESITE-DIORITE  SERIES. 
THE  ANDESITES. 

SiO,  AlaO,  Fe,0,  FeO  CaO  MgO  Na.O  K2O  Loss 

1.  67.83  15.02            ...  5.16  3.07  0.29  2.40  3.20  I. II 

2.  65.50  14.94  1.72  2.27  2.33  2.97  5.46  2.76  1.37 

3.  63.49  18.40  2.44  1.09  2.30  0.66  5.70  4.62  1.04 

4.  62.94  18.14  ...  3.82  6.28  3.06  3.83  1.22  O.6O 

5.  61.62        16.86          ...          6.61        6.57        2.07        3.93        1.66 

6.  61.58        16.34          ...          6.42        5.13        2.85        2.69        3.65  0.64 

7.  59.48        16.37        3.21        3.17        4.88        3.29        3.30        2.81  2.02 

8.  56.19        16.12        4.92        4.43        6.99        4.60        2.96        2.37  1.03 

9.  56.91         18.18        4.65        3.61        7.11        3.49        4.02        1.61  0.36 
I.  Hb.-mica-andesite,  Eureka  Dist.,  Nev.,  Mono.  XX.,  U.  S.  G.  S.,  264.    2.  Hb.- 

mica-andesite,  Sepulchre  Mountain,  Yellowstone  Park,  J.  P.  Iddings,  Phil.  Soc. 
Wash.,  XI  ,  210.  3.  Mica-andesite,  Rosita  Hills,  Colo.,  W.  Cross,  Colo.  Sci. 
Soc.,  1887,  250.  4.  Lassen's  Peak,  Calif.,  Hague  and  Iddings,  A.  J.  S.,  Sept., 
1883,  225.  5.  Mt.  Rainier.  See  last  reference.  6.  Pyroxene-andesite,  Eureka  Dist., 
Nev.,  Mono.  XX.,  U.  S.  G.  S.,  264.  7.  Hypersthene-andesite,  near  Red  Bluff, 
Mont.,  G.  P.  Merrill,  Proc.  U.  S.  Nat'l  Museum,  XVII.,  651.  8.  Hypersthene- 
andesite,  Buffalo  Peaks,  Colo.,  W.  Cross,  Bull.  I.,  U.  S.  G.  S.,  26.  9.  Colombia,  S. 
America. 

Comments  on  the  Analyses. — It  appears  at  once  from  the  analyses 
that  the  andesites  lap  over  the  lower  limits  of  the  dacites  and 
have  much  the  same  range  in  silica  as  the  trachytes.  All  the 
bases  reach  notable  percentages,  but  the  alkalies  recede  as  the 
others  increase. 

As  compared  with  both  dacites  and  trachytes  these  contrasts 
are  well  brought  out  in  the  diagrams,  Figs.  24  and  25.  The  in- 
tercepts of  silica  shorten,  whereas  those  of  lime  and  magnesia  not- 
ably lengthen. 


IGNEOUS  ROCKS. 


61 


General  Description.  —  The  Andesite  series  embraces  a  large  and 
wide-spread  group  of  rocks,  which  marks  an  important  step  from 
the  more  acidic  to  the  more  basic  limits  of  the  igneous  types.  Its 
members  are  emphatically  rocks  of  medium  acidity,  with  the  light- 
colored  minerals  still  in  excess  over  the  dark-colored  ones.  The 
feldspars  are  therefore  the  most  prominent  components  but  there 
is  a  marked  increase  in  the  ferromagnesian  silicates  as  compared 
with  the  dacites.  Quartz  fails  except  as  a  rare  and  sporadic  com- 
ponent. Hornblende  and  augite  begin  to  take  precedence  over 


FIGS.  24  AND  25.  Diagrams  illustrating  the  chemical  composition  of  the  andesites, 
based  on  the  above  analyses.  Fig.  24  refers  to  molecular  ratios ;  Fig.  25  to  per- 
centages. 

biotite,  but  all  three  are  common.  The  prevailing  colors  are 
grays  or  greens,  mottled  by  the  light  and  dark  phenocrysts.  The 
andesites  have  fusing  points  near  2000°  F.  (iiOO°  C).  They 
rarely  afford  large  amounts  of  glasses. 

The  textures  of  the  Andesite  series  range  from  felsitic  to  coarsely 
porphyritic.  The  Andesites  proper  are  felsitic  or  moderately  por- 
phyritic  rocks,  sometimes  cellular  from  their  crystallization  as 
surface  flows.  When  finely  felsitic  they  cannot  be  readily  distin- 
guished from  the  trachytes  and  even  from  the  dacites  and  rhyolites 
of  the  same  texture,  although  they  are  usually  provided  with  more 
of  the  dark  silicates  than  are  the  last  two.  For  sharp  determina- 
tion recourse  must  be  had  to  the  microscope,  but  it  is  fair  to  men- 


62  A   HAND  BOOK  OF  ROCKS. 

tion  that  andesites  are  much  more  abundant  rocks  in  Nature  than 
are  trachytes,  so  that  in  doubtful  cases  the  chances  strongly  favor 
the  former.  When  examined  with  the  microscope  the  finely 
crystalline  groundfnasses  of  the  andesites  are  often  found  to  be  a 
fine  felt  of  little  rods  of  feldspar,  giving  a  texture  that  is  fairly 
characteristic  of  this  group. 

With  the  development  of  phenocrysts  the  exact  determination 
of  the  andesites  becomes  less  difficult.  Quartz  fails  and  the  feld- 
spars constitute  the  more  prominent  porphyritic  crystals.  The 
cleavage  faces  of  the  latter  should  then  be  examined  with  a  lense 
and  if  the  greater  number  exhibit  the  characteristic  striations  of 
plagioclase  the  andesites  may  be  recognized  as  against  the  tra- 
chytes. This  determination  may  be  further  fortified  by  the  fre- 
quent greater  prominence  of  the  dark  silicates.  The  andesites 
proper  occur  characteristically  in  surface  flows. 

As  the  phenocrysts  become  abundant  the  andesites  proper  pass 
into  the  Andesite-porphyries.  The  groundmass  is,  as  a  rule,  fel- 
sitic.  The  cellular  texture  disappears  entirely  and  the  rocks  are 
dense  and  markedly  porphyritic.  The  interiors  of  thick  surface 
flows,  the  dikes  and  intrusive  sheets  and  the  outer  parts  of  lacco- 
liths are  their  special  home. 

When  phenocrysts  are  in  marked  excess  over  the  groundmass 
and  constitute  the  greater  part  of  the  rock  the  Diorite-porphyries 
result.  The  groundmass  is  rather  coarsely  felsitic  and  becomes 
increasingly  so  as  the  diorites  are  approached.  All  the  minerals 
forming  the  phenocrysts  are  now  not  difficult  to  recognize.  The 
diorite-porphyries  occur  in  deep-seated  dikes,  thick  intrusive  sheets 
and  the  central  parts  of  laccoliths.  They  mark  a  textural  transi- 
tion to  the  diorites. 

Synonyms  and  Relatives.  —  The  name  andesite  was  first  proposed 
by  L.  von  Buch  in  1835  for  certain  lavas  from  the  Andes  Moun- 
tains which  consisted  of  albite  and  hornblende,  and  which  there- 
fore differed  from  trachyte  in  the  old  sense.  The  name  did  not 
come  into  general  use  until  1858  since  which  time  it  has  been 
quite  universally  employed  for  the  porphyritic  and  felsitic  plagio- 
clase-bearing  eruptives  of  medium  acidity.  For  a  time  it  was  re- 
stricted to  the  Tertiary  and  later  rocks  but  this  limitation  is  no 
longer  current  and  textural  features  are  alone  emphasized.  Ande- 


IGNEOUS  ROCKS.  63 

sites  whose  chief  dark  silicate  is  biotite  are  called  mica-andesites ; 
those  with  hornblende,  hornblende-  or  amphibole-andesites  ;  while 
those  with  augite  are  known  as  augite-andesites.  Hypersthene- 
andesites  are  occasionally  met  and  result  when  the  magma  is  rich 
in  magnesia.  The  augite-andesites  differ  from  the  olivine-free 
basalts  in  having  the  light-colored  minerals  in  excess. 

While  the  time-distinction  was  still  preserved  in  the  classification 
of  igneous  rocks,  the  pre-Tertiary  andesites  were  called  by  some 
porphyrite,  to  which  name  the  several  prefixes,  mica,  hornblende 
and  augite  were  attached.  Later  porphyrite  was  employed  for  the 
deep-seated  or  intrusive  andesites,  which  are  here  called  andesite- 
porphyry  and  diorite-porphyry,  but  even  this  use  is  practically 
obsolete  as  it  is  certainly  unnecessary.  Other  rock-names  more 
or  less  closely  related  to  andesite,  such  as  asperite,  propylite, 
volcanite  and  latite  will  be  found  defined  in  the  Glossary. 

Andesites,  with  the  increase  of  orthoclase  and  the  corresponding 
decrease  of  plagioclase,  pass  into  the  trachytes;  and  with 
the  increase  of  the  dark  silicates  and  corresponding  decrease  of 
feldspar  they  shade  into  the  basalts.  The  appearance  of  quarto 
in  notable  amounts  marks  a  transition  to  the  dacites.  Practic- 
ally unbroken  series  can  easily  be  selected  to  all  these  related 
groups. 

Alteration,  Metamorphism.  —  The  andesites  in  decay  afford  kaoli- 
nized  material  and  mixtures  of  this  with  chloritic  products  that  are 
very  difficult  to  identify.  Thus  the  now  famous  andesitic  breccia 
at  Cripple  Creek,  Colo.,  can  rarely  be  shown  to  the  eye  to  be  other 
than  a  white,  kaolinized  mass,  and  decomposed  outcrops  of  massive 
flows  are  no  less  unsatisfactory.  Where  metamorphic  processes 
affect  older  flows,  felsitic  and  silicified  forms  result  similar  to  those 
mentioned  under  rhyolites.  The  tracing  of  the  history  of  the  rock 
is  then  a  matter  for  the  microscope  and  chemical  analysis  when 
indeed  it  can  be  done. 

Tuffs.  —  Andesitic  tuffs  and  breccias  (i.  e.,  aggregates  of  angular, 
volcanic  ejectments  coarser  than  tuffs)  are  rather  common  in  the 
western  volcanic  districts.  With  ordinary  observation  they  can 
only  be  identified  by  finding  fragments  large  and  fresh  enough  to 
indicate  the  original.  Such  have  proved  of  great  economic  im- 
portance at  Cripple  Creek,  Colorado. 


64  A  HAND  BOOK  OF  ROCKS. 

Distribution. — Andesites  are  very  wide-spread  in  the  West. 
The  vast  laccoliths  that  form  many  of  the  peaks  in  Colorado  are 
intruded  andesite-porphyries  of  a  rather  acidic  type,  frequently 
with  some  orthoclase.  In  the  Yellowstone  Park  they  are  impor- 
tant. In  Nevada,  as  at  Eureka  and  the  Comstock  lode,  they  have 
proved  of  great  geological  interest,  and  especially  in  and  near  the 
latter,  with  its  many  miles  of  drifts,  shafts  and  tunnels,  very  impor- 
tant data  for  the  study  of  rock  masses  have  been  afforded.  The 
old  cones  along  the  Pacific,  Mt.  Hood,  Mt.  Shasta,  Mt.  Rainier  and 
others  are  chiefly  andesite.  The  products  of  Mexican  and  South 
American  volcanoes  are  also  of  this  type,  and  indeed  along  the 
whole  Pacific  border  the  recent  lavas  have  many  features  in  common. 
Abroad  andesites  are  seldom  lacking  in  great  volcanic  districts. 

THE  DIORITES. 

SiO,  AlaO,       Fe.O.        FeO         CaO         MgO      Na.O       K,O        Loss.      Sp.  Gr. 

1.  61.75         18.88         0.52        3.52        3.54        1.90       3.67        1.24       4.46       2.79 

2.  58.05          18.00         2.49        4.56        6.17        3.55        3.64        2.18        086 

3.  56.71       18.36        ...        6.45      6.ii      3.92      3.52      2.38       ...        2.86 
4-        52-35       I5-72       2-9°      7-32      8.98      7.36      2.81       1.32      1.35 

5.  50.47       18.73       4- '9      4-92      8.82      3.48      4.62      3.56      0.58      2.87 

6.  48.98       17.76       2.14      6.52      8.36      2.09      6.77      2.08      4.50 

7.  48.19       16.79     18.37        ...        6.85      1.32      5.59      1. 1 1      2.31 

I.  Diorite.  Pen-maen-mawr,  Wales,  J.  A.  Phillips,  Q.  J.  G.  S.,  XXXIIL,  424, 
1877.  2.  Diorite,  Electric  Peak,  Yellowstone  Park,  J.  P.  Iddings,  Bull.  Phil.  Soc. 
Washington,  II.,  206.  3.  Diorite  (granitoid  andesite?),  Comstock  Lode,  Nev.,  R.  W. 
Woodward,  4Oth  Par.  Survey,  I.,  opp.  p.  676.  4.  Augite-diorite,  Little  Falls,  Minn., 
A.  Streng,  Neues  Jahrb.,  1877,  129.  5.  Augite-diorite,  Mt.  Fairview,  Custer  Co., 
Colo.,  W.  Cross,  Anal,  by  Eakins,  Col.  Sci.  Soc.,  1887,  247.  6.  Porphyritic-diorite, 
St.  John,  N.  B.,  W.  D.  Matthew,  Trans.  N.  Y.  Acad.  Sci.,  XIV.,  213.  7.  Diorite 
dike  rich  in  magnetite,  Forest  of  Dean  Mine,  N.  Y.,  J.  F.  Kemp,  A.  J.  S.,  Apr., 
1888,  331. 

Comments  on  the  Analyses.  —  As  regards  silica  the  analyses  be- 
gin where  those  of  the  quartz-diorites  leave  off,  and  extend  to  lower 
limits  than  those  of  the  andesites.  The  last  three  are  indeed  more 
basic  than  is  typical  of  the  diorites.  The  bases,  iron,  lime  and 
magnesia  show  a  marked  increase,  but  in  the  typical  cases  do  not 
yet  reach  the  figures  of  the  basaltic  rocks  which  follow.  Soda  is 
in  excess  over  potash  as  would  follow  from  the  prevalence  of 
plagioclase. 

Figs.  26  and  27  are  of  special  interest  when  compared  with 
those  of  the  andesites  which  they  greatly  resemble.  The  diorites 


IGNEOUS  ROCKS.  65 

show  a  slight  increase  in  the  intercepts  above  the  horizontal,  a 
slight  decrease  in  silica  and  in  potash,  but  in  other  respects  they 
are,  as  they  ought  to  be,  very  much  the  same  as  Figs.  24  and  25. 


FIGS.  26  AND  27.  —  Diagrams  illustrating  the  chemical  composition  of  the  diorites, 
which  are  given  in  the  above  analyses.  Fig.  26  is  based  on  molecular  ratios  ;  Fig.  27 
on  percentages. 

Mineralogical  Composition.  —  The  diorites  are  granitoid  igneous 
rocks,  whose  chief  feldspar  is  plagioclase  and  whose  chief  dark 
silicate  is  either  hornblende  or  biotite.  Those  with  hornblende 
are  called  simply  diorites  ;  those  with  biotite,  mica-diorites.  Some 
augite  is  often  present,  marking  a  passage  to  the  gabbros  and  giv- 
ing the  rock  the  name  augite-diorite.  It  is  however  a  matter  of 
much  difficulty  to  distinguish  hornblende  from  augite  with  the  eye 
alone,  and  unless  the  observer  can  make  certain  of  the  cleavages  — 
approximately  120°  for  hornblende,  and  90°  for  augite  —  doubt 
may  arise.  In  the  typical  diorites  the  feldspars  are  in  excess  over 
the  dark  silicates  and  contrasts  are  thus  afforded  with  the  typical 
gabbros,  but  the  name  diorite  in  ordinary  use  is  often  applied  to 
rocks  with  a  decided  excess  of  hornblende.  Additional  difficulty 
in  the  sharp  application  of  the  word  arises  because  under  the  influ- 
ence of  metamorphism  original  augite,  as  for  instance  in  a  gabbro, 
changes  readily  in  whole  or  in  part  to  hornblende,  and  a  mineral- 
5 


66  A  HAND  BOOK  OF  ROCKS. 

ogical  aggregate  thus  results  which  corresponds  to  diorite,  yet 
which  did  not  crystallize  directly  in  this  form.  When  working 
with  the  microscope  the  observer  can  follow  out  these  changes,  but 
when  depending  on  the  eye  alone,  it  is  necessary  to  base  the 
determination  on  the  minerals  as  we  find  them. 

Magnetite,  titanite  and  apatite  are  almost  always  present  as 
accessory  minerals  in  the  diorites,  but  are  usually  too  small  to  be 
seen.  Garnet  is  not  infrequent,  and  pyrrhotite  at  times  is  in  con- 
siderable amount. 

The  name  diorite  was  first  applied  in  1822  by  the  Abbe  Haiiy. 
It  is  derived  from  the  Greek  verb,  to  distinguish,  and  was  sug- 
gested by  the  fact  that  in  the  rocks  first  named  the  white  feldspar 
could  be  easily  distinguished  from  the  black  hornblende.  In  the 
course  of  time  it  became  a  very  widely  used  field  name  among 
geologists  and  miners. 

As  noted  in  the  definition  of  diorite  in  the  Glossary  there  is  a 
decided  disposition  among  students  of  rocks,  especially  when 
working  with  the  microscope,  to  apply  the  name  diorite  to  those 
varieties  of  the  plagioclase  rocks  whose  feldspar  is  more  acidic 
than  labradorite;  that  is  when  the  plagioclase  comes  within  the 
albite-andesine  ranges.  The  dark  silicate  may  then  be  horn- 
blende, biotite  or  pyroxene.  The  rocks  with  the  more  basic 
plagioclases  are  classed  with  the  gabbros.  Without  the  micro- 
scope, these  distinctions  among  the  plagioclases  are,  however, 
impracticable. 

Varieties.  —  The  varieties  mica-diorite  and  augite-diorite  have 
already  been  defined.  A  dioritic  rock  occurring  in  dikes  and  con- 
taining both  hornblende  and  biotite  has  been  named  kersantite. 
Camptonite  is  applied  to  a  rock,  found  in  dikes  which  are  often 
met  in  close  association  with  nephelite-syenite  and  which  have  the 
composition  of  hornblende-diorite.  Additional  details  regarding 
both  these  as  well  as  augite-diorite  and  definitions  of  banatite, 
vogesite,  and  kersanton  will  be  found  in  the  Glossary. 

Alteration,  Metamorphism. — In  ordinary  alteration  the  feldspar  of 
diorites  kaolinizes  and  the  hornblende  changes  to  chlorite,  affording 
one  of  the  varieties  of  the  so-called  greenstones.  Under  shearing 
stresses  in  metamorphism  the  diorites  pass  into  gneisses,  and  into 
hornblende  schists  or  amphibolites.  In  many  mining  regions  even 


IGNEOUS  ROCKS.  67 

decidedly  schistose  varieties  are  still  called  diorite.  A  final  stage  is 
chlorite-schist,  wherein  the  hornblende  has  altered  to  chlorite. 

Distribution. — True,  original  diorites  are  not  very  common  rocks 
in  America.  In  the  Sudbury  nickel  district,  north  of  Lake  Huron, 
dense,  dark  diorites  are  the  chief  rock  containing  the  ore,  but  there 
is  always  the  possibility  that  the  hornblende  is  altered  augite.  Mt. 
Davidson,  above  the  Comstock  Lode,  is  either  a  true  diorite  or  a 
granitoid  phase  of  andesite.  Authorities  differ  as  to  its  interpretation. 

Diorites  are  well  known  abroad  and  have  been  described  from 
various  places  in  Great  Britain,  Germany,  France  and  Austria. 

The  Dacite- Diorite  Table.  —  Regarding  the  recasting  of  these 
analyses  the  general  remarks  given  on  p.  38  also  apply.  With 
the  increase  of  the  dark  silcates  additional  difficulties  arise  in  that 
the  A12O3  is  not  so  largely  limited  to  the  feldspars.  The  chief 
chemical  difference  as  compared  with  the  rocks  already  discussed 
is  the  increase  in  the  CaO.  It  brings  about  the  great  advance  in 
the  anorthite  and  contributes  to  the  amphibole.  Under  amphibole 
some  pyroxene  may  also  be  understood  as  possible.  It  is  interest- 
ing to  note  that  quartz  must  be  assumed  in  all  the  analyses  of 
andesites,  although  in  most  of  them  the  silica  appears  scarcely  to 
suggest  it.  Orthoclase  appears  in  every  analysis.  With  one 
exception  (64.4)  albite  is  the  most  abundant  molecule.  Anorthite 
ranges  from  a  minimum  of  7.7  to  a  maximum  of  27,  high  values 
compared  to  the  rocks  already  passed  in  review.  The  light- 
colored  minerals  vary  in  total  from  a  maximum  of  96  to  a 
minimum  of  65.2,  always  using  volumes.  The  dark-colored  ones 
range  from  4  to  34.8.  They  are  chiefly  amphibole,  sometimes 
with  biotite,  olivine  and  pyroxene,  which  last  was  not  specially 
calculated.  Magnetite  is  always  in  evidence  and  reaches  abnormal 
values  in  the  last  analysis,  which  was  made  of  a  dike,  where  it  cut 
a  large  ore-body  of  magnetite. 


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CHAPTER  V. 

THE  IGNEOUS  ROCKS,  CONTINUED.     THE  BASALT-GABBRO  SERIES. 

THE  FELDSPAR-FREE  BASALTS.     THE  PYROXENITES  AND 

PERIDOTITES.     THE  ULTRA-BASIC  ROCKS. 

THE  BASALT-GABBRO   SERIES. 
THE  BASALTS. 

Basalts.  SiO,         AlaOs       Fe,O3      FeO          CaO        MgO       Na,O       KaO        Loss.     Sp.  Gr. 

1.  57.25      16.45      1-67      i-77        7-65      6.74      3.00      1.57      0.45 

2.  53.81         13.48        3-02         7.39         10.34        6.46        3.23        0.64        0.57        2-75 

3.  53.62      22.09      4.21        ...          6.02      6.24      3.16      0.57      5.03 

4.  52.27         17.68        2.51         5-00  8.39        6.05         4.19         1.58        0.82 

5.  51.58         11.92         2.96      13.05  8.52        4.09        0.95         0.34         1.52        2.989 

6.  50.38      19.83      6.05      2.00      10.03      5-3^      2.15      1.76      1.37 

7.  49.45     17.58     3.41     3.41      7.20    4.05     5.83     1.57     4.34 

8.  49-04      i8.ii      2.71      7.70        7.11      4.72      4.22      2.11      1.29      2.738 

9.  48.40      17.95      2.28      8.85       10.05      6.99      2.86      1.03      0.34      2.8 

10.  47-54       19.52      4.24      6.95       11.70      6.66      3.09      0.16  2.981 

11.  46.43       17.10     11.16        ...        10.38      9.78      2.50        ...        2.65 

I.  Basalt  with  quartz,  Cinder  Cone,  Calif.,  J.  S.  Diller,  A.  J.  S.,  Jan.,  1887,  p.  49, 
Anal.  Hillebrand.  2.  Kilauea,  Sandwich  Is.;  Cohen.,  Neues  Jahrb.,  1880,  II.,  41. 
3.  Iceland,  Schirlitz,  Tsch.  Mitth.,  1882,  440.  4.  Rio  Grande  Canon,  N.  M.,  J.  P. 
Iddings,  A.  J.  S.,  Sept.,  1888,  220,  Anal.  Eakins.  5.  Dalles,  Oregon,  Lemberg,  Z. 
d.  d.  g.  G.,  XXXV.,  116.  6.  Richmond  Mtn.,  Eureka  Dist,  Nev.,  A.  Hague,  Mono. 
XX.,  U.  S.  G.  S.,  264,  Anal.  Whitfield.  7.  Point  Bonita,  Calif,  F.  L.  Ransome, 
Bull.  Geol.  Dept.  Univ.  Calif.,  I.,  106.  8.  Buffalo  Peaks,  North  Park,  Colo., 
Woodward,  4Oth  Parallel  Surv.,  II.,  126.  9.  Shoshone  Mesa,  Nev.,  Woodward,  4Oth 
Par.  Surv.,  II.,  617.  10.  Cascade  Mts.,  Oregon,  Jannasch,  Tsch.  Mitth.,  1881,  102. 
II.  Glassy  basalt,  Edgecombe  Island,  near  Sitka,  Alaska,  Lemberg,  Z.  d.  d.  g.  G., 
XXXV.,  570. 

Comments.  —  The  first  analysis  is  very  like  the  more  basic  ande- 
sites,  except  in  its  high  percentage  of  MgO.  It  is  of  a  curious  and 
exceptional  basalt  with  quartz  phenocrysts,  regarding  which  men- 
tion is  made  later.  In  general,  the  others  are  notably  high  in  the 
oxides  of  iron,  in  CaO  and  MgO.  The  alkalies  wane  because  of 
the  increasing  inferiority  of  the  feldspars,  which  give  place  to 
augite  and  olivine.  The  specific  gravity  is  high. 

The  diagrams,  Figs.  28  and  29,  present  interesting  contrasts 
with  all  which  have  preceded.  The  intercepts  for  silica  have 

69 


70  A  HAND  BOOK  OF  ROCKS. 

drawn  in,  because  of  the  increasing  basicity.  The  alumina  has  de- 
cidedly shrunk  as  have  the  alkalies,  indicating  the  waning  amounts 
of  the  feldspars.  The  iron,  lime  and  magnesia  on  the  contrary  are 
much  greater,  and  thus  emphasize  the  increase  of  the  dark  silicates. 


FlGS.  28  AND  29.  Diagrams  illustrating  the  chemical  composition  of  the  basalts, 
which  are  given  in  the  above  table  of  analyses.  Fig.  28  is  based  on  molecular  ratios  ; 
Fig.  29  on  percentages. 

General  Description. — The  Basalt  series  marks  a  decided  step 
toward  the  basic  extreme  of  the  igneous  rocks.  The  dark-colored 
silicates  are  now  in  excess  and  give  the  chief  characters  to  the 
rocks.  Augite  and  olivine  are  the  ones  of  greatest  importance, 
hornblende  being  very  rare,  and  biotite  scarcely  known.  The 
feldspars  appear  as  the  more  basic  plagioclases,  labradorite  to 
anorthite.  Magnetite  is  a  prominent  component  although  usually 
too  small  to  be  recognized  by  the  eye  alone.  The  prevailing  colors 
of  the  basalts  are  dark  grays  and  blacks.  When  red  or  green 
the  color  is  due  to  the  advance  of  alteration.  The  fusing  points 
are  comparatively  low,  being  in  the  neighborhood  of  1940°  F. 
(1060°  C).  Glassy  varieties  are  rare,  although  slaggy  crusts  and 
scorias  are  not  unusual.  The  tendency  to  crystallize  has  been 
marked  and  irresistible  during  the  process  of  cooling. 


IGNEOUS  ROCKS.  71 

The  textures  are  much  more  commonly  felsitic  than  with  the 
other  groups,  but  porphyritic  varieties  are  often  met.  The  Basalts 
proper  are  felsitic,  or,  rather  rarely,  somewhat  porphyritic  rocks, 
and  often  cellular  because  of  their  crystallization  as  surface  flows. 
The  crusts  upon  these  streams  of  basic  lava  may  be  very  scori- 
aceous  because  of  the  free  expansion  of  the  dissolved  vapors  but 
from  a  point  not  far  below  the  surface  and  thence  to  the  interior, 
the  rock  becomes  increasingly  dense.  Such  cellular  development 
as  has  taken  place  is  then  manifested  in  scattered,  rounded  or 
almond-shaped  cavities  called  amygdaloids  from  the  Greek  word 
for  an  almond,  whose  outlines  they  closely  resemble.  These 
cavities  frequently  become  filled  with  secondary  calcite,  quartz  or 
chlorite,  but  the  rounded  masses  must  not  be  mistaken  for  pheno- 
crysts.  The  under  as  well  as  the  upper  portions  of  basaltic  flows 
are  provided  with  them  and  in  the  copper  region  of  Lake  Superior 
they  attain  decided  importance,  because  they  have  occasionally 
furnished  a  place  of  deposition  for  copper. 

Basalts  are  frequently  dense  and  felsitic  and  exhibit  no  pheno- 
crysts  whatever.  They  must  then  be  recognized  by  their  dark 
colors,  and  high  specific  gravity.  When  the  phenocrysts  manifest 
themselves  they  are  most  commonly  olivine  with  which  augite  may 
at  times  be  recognized.  If  studied  with  the  microscope  the  felsitic 
varieties  are  resolved  into  a  finely  crystalline  mass  of  small  augites, 
plagioclase  rods  and  magnetites.  A  little  glass  may  occasionally 
be  detected. 

With  the  development  of  phenocrysts  in  moderate  numbers  the 
basalts  proper  pass  into  the  Basalt-porphyries,  often  called  dolerites. 
Phenocrysts  of  augite  and  olivine  become  prominent  features  of  the 
rock,  but  visible  plagioclases  are  few.  The  cellular  texture  disap- 
pears and  the  rocks  are  dense  and  solid.  The  basalt-porphyries 
constitute  the  interiors  of  thick  flows  and  often  form  dikes.  Yet  it 
happens  much  more  frequently  with  these  basaltic  rocks  than  with 
those  more  acidic,  that  the  interiors  of  thick  flows,  instead  of  be- 
coming porphyritic,  develop  coarser  and  coarser,  even  textured 
varieties  which  must  be  treated  with  the  granitoid  members. 

When  phenocrysts  are  in  marked  excess  over  the  groundmass 
and  constitute  the  greater  part  of  the  rock,  the  Gabbro-porphyries 
result.  The  groundmass  is  then  coarsely  felsitic  and  becomes  in- 


72  A  HAND  BOOK  OF  ROCKS. 

creasingly  coarse  as  the  gabbros  are  approached.  These  rocks 
occur  in  deep-seated  dikes  and  in  the  outer  parts  of  laccoliths  and 
thick  intrusive  sheets. 

Synonyms  and  Relatives.  —  The  name  basalt  is  a  very  ancient 
term  and  has  been  explained  in  several  ways.  Many  regard  it  as 
a  corruption  of  basanites,  which  was  used  by  Pliny,  although  it  is 
uncertain  to  what  rock  he  applied  it.  The  Greek  word  for  the 
black  touchstone  or  Lydian  stone  used  by  the  ancient  jewellers  is 
similar  to  this  last  form.  Others  refer  it  to  Basan  or  Bashan,  the 
kingdom  of  Og,  as  mentioned  in  the  Old  Testament,  Deuteronomy 
III.,  I.  Again  an  Ethiopian  word  "basal,"  used  by  Pliny  for  an 
iron-bearing  rock,  has  been  suggested.  Agricola  in  the  sixteenth 
century  gave  it  its  present  signification. 

The  basalts  almost  always  have  olivine  as  an  essential  compo- 
nent, but  there  are  certain  rather  uncommon  varieties  which  lack  it, 
but  which  still  have  the  dark  silicate,  augite,  in  excess  over  the 
plagioclose,  and  which  therefore  differ  from  the  andesites.  They 
are  called  olivine-free  basalts,  and  mark  a  transition  to  the  augite- 
andesites.  Very  rarely  indeed,  hornblende  replaces  the  augite  in 
basalts  and  then  the  rocks  are  called  kulaites  from  the  occurrence 
of  this  variety  in  the  Kula  basin,  Lydia,  Asia  Minor,  where  they 
have  been  discovered  and  studied  by  H.  S.  Washington. 

While  the  time-distinction  was  esteemed  of  weight  by  the 
students  of  rocks,  the  name  basalt  was  restricted  to  those  of  Terti- 
ary or  later  age.  The  pre-Tertiary  representatives  were  then  called 
augite-porphy rites  if  they  lacked  olivine,  and  melaphyre  if  they 
possessed  it.  Melaphyre  still  survives  as  a  much  used  term  but 
the  time  distinction  no  longer  obtains  recognition. 

In  rare  instances  nephelite  and  leucite  are  found  in  basalts  when 
studied  with  the  microscope.  The  two  feldspathoids  may  appear, 
each  by  itself  or  both  together  and  they  may  replace  the  plagioclase 
in  part  or  in  whole.  These  varieties  cannot  be  distinguished  from 
normal  basalts  without  microscopic  study.  They  have  been  named 
tephrite,  leucite-tephrite,  basanite,  leucite  basanite,  nephelinite, 
leucitite,  nephelite-basalt  and  leucite-basalt,  all  of  which  will  be 
found  defined  in  the  Glossary.  It  is  fruitless  to  attempt  their  study 
without  the  microscope.  The  most  experienced  observers  might 
easily  confound  them  with  ordinary  basalts. 


IGNEOUS  ROCKS.  73 

At  several  localities  in  America  and  abroad  a  very  extraordinary 
and  abnormal  variety  of  basalt  has  been  met  which  has  quartz, 
even  as  a  visible  phenocryst.  This  mineral  has  resulted  either 
from  some  extraordinary  circumstances  attending  early  crystalliza- 
tion, so  that  quartz  developed  as  a  phenocryst,  or  else  because 
with  a  percentage  of  silica  at  the  upper  ranges  of  the  basalts,  the 
ferromagnesian  bases  were  in  such  amount  as  to  leave  an  excess 
of  silica  after  their  basic  compounds  were  formed.  This  residual 
silica  then  crystallized  as  quartz  since  it  had  no  alternative.  These 
chemical  relations  are  however  extremely  unusual,  and  the  rocks 
are  exceptional  in  the  highest  degree. 

Two  other  varieties  of  basalts  may  be  mentioned  at  this  point 
and  at  somewhat  greater  length  because  of  their  special  minera- 
logical  and  chemical  interest  and  their  relations  to  corresponding 
granitoid  types.  Neither  of  them  can  be  recognized  with  the  eye 
alone  as  differing  in  any  respect  from  the  ordinary  basalts,  but 
when  studied  with  the  microscope  they  present  great  contrasts. 
In  both,  feldspars  and  feldspathoids  practically  or  absolutely  fail. 
We  have  left  then  the  augitites,  which  consist  of  augite  in  a  glassy 
groundmass,  and  the  limburgites  which  have  both  augite  and 
olivine  in  a  similar  groundmass.  As  the  analyses  will  show  these 
mineralogical  results  become  possible  when,  with  very  low  silica, 
the  alkalies  and  alumina  so  far  fail  that  they  are  taken  up  in  the 
dark  silicates  or  glassy  base. 

AUGITITES. 

SiO,         A130,      Fe.,0,       FeO          CaO         MgO       Na4O       K,O         Lois.     Sp.  Gr. 

1.  44- 17       11-24      9-97       6.22       10.77       655       3.04       1.97       2.31 

2.  43.35       11.46     11.98      2.26        7.76     11.69      3-88      0.99      3.00      2.974 

LlMBURGITE. 

3.  42.06       12.18       2.67       7.89        11.29       "-47     5-1°       L07       3.08       2.968 

4.  40.22        14.41        7.42       2.36       11.53         7.29     3-94       1.9°       ».l°      2.89 
I.   Augitite,  Mariupol,  Russia,  J.  Morozewicz,  Neues  Jahrb.,  1900,  I.,  394.     Also 

TiO,  2.83.  2.  Augitite,  Hutberg,  Bohemia,  J.  E.  Hibsch,  Tsch.  Mitth.,  XIV.,  no, 
1894.  Also  TiO,  2.43,  pi°s  1'54-  3-  Limburgite,  Hahn,  Hesse- Nassau,  P.  Jannasch, 
Sb.  Berl.  Akad.  1889,  p.  1026 ;  also  TiO,  1.93,  P,OP  34.  4.  Limburgite,  Palma, 
L.  Van  Werveke,  Neues  Jahrb.  1879,  485. 

Figs.  30  and  31  are  of  special  interest  when  compared  with 
those  of  the  basalts,  Figs.  28  and  29.  The  great  diminution  in 
the  intercept  of  alumina  and  the  great  increase  in  lime  and  mag- 


74 


A  HAND  BOOK  OF  ROCKS. 


nesia  are  very  marked.     The  diagrams  strongly  resemble  those 
of  the  pyroxenites  and  peridotites,  given  subsequently  under  Figs. 

35-38. 

There  is  some  question  in  the  minds  of  many  observers  as  to 
whether  the  so-called  glassy  base  of  the  augitites  and  limburgites 
really  is  a  glass  or  whether  it  is  not  some  isotropic  mineral  such 
as  analcite.  The  basic  magmas  crystallize  so  readily  as  to  make 
so  much  glass  improbable.  Analcite  is  the  mineral  usually 
thought  of  in  this  connection.  It  even  gives  a  name  to  certain 
analcite-basalts,  in  which  its  presence  has  been  positively  shown. 


c»o 


FIGS.  30  AND  31.  Diagrams  illustrating  the  chemical  composition  of  the  lim- 
burgites which  are  given  in  the  above  table.  Fig.  30  is  based  on  molecular  ratios ; 
Fig.  31  on  percentages. 

The  related  rocks  monchiquite,  fourchite  and  ouachitite,  will  be 
found  defined  in  the  Glossary. 

A  very  rare  basaltic  rock  has  the  feldspathoid  melilite  as  its 
chief  feldspathic  component.  The  magma  is  high  in  lime.  The 
rock  can  only  be  identified  with  the  microscope.  Its  related  type 
alnoite  is  defined  in  the  Glossary. 

Alteration,  Metamorphism.  —  The  olivine  of  basaltic  rocks  is  the 
first  mineral  to  alter,  and  it  soon  becomes  a  network  of  serpentine 


IGNEOUS  ROCKS.  75 

veinlets  enclosing  unchanged  nuclei.  The  augite  also  passes 
readily  into  chlorite  and  finally  the  feldspar  kaolinizes.  The  preva- 
lence of  green,  chloritic  products  suggested  the  name  greenstone 
for  the  old  basaltic  rocks.  The  basaltic  rocks  are  extremely  im- 
portant in  connection  with  metamorphism,  and  the  iron-mining 
regions  around  Lake  Superior  present  superb  illustrations  of  the 
process.  The  augite  has  the  greatest  tendency  to  pass  into  green 
hornblende,  by  what  is  called  a  "  paramorphic "  change,  i.  e.,  a 
change  in  the  mineral  without  change  in  the  chemical  composition 
and  without,  as  in  pseudomorphs,  preserving  the  original  form. 
Under  shearing  stresses  and  movements,  accompanied  by  this 
paramorphic  change,  the  basaltic  rocks  pass  into  hornblende- 
schists,  and  even  chlorite-schists  or  green-schists,  losing  their  mas- 
sive structure  entirely  and  becoming  a  very  different  rock,  and  one 
that  can  be  traced  to  its  original  with  great  difficulty.  The  wide- 
spread Catoctin  schists  of  Virginia  were  derived  in  this  way.  The 
secondary  hornblendic  rocks  are  also  called  amphibolites. 

Tuffs.  —  Basaltic  tuffs,  agglomerates,  breccias,  etc.,  are  well 
known  and  often  accompany  the  massive  flows.  They  mark  an 
explosive  stage  of  eruption  before  or  after  the  actual  outpouring  of 
lava. 

Distribution.  —  Basaltic  rocks  are  enormously  developed  in  this 
country.  The  oldest  strata  are  penetrated  by  numerous  black, 
igneous  dikes,  in  practically  all  their  exposures.  The  New  Eng- 
land seacoast  is  especially  seamed  by  them,  and  hundreds  may  be 
met  in  a  short  distance.  The  Adirondacks  and  the  White  Moun- 
tains, the  Highlands  of  New  York  and  New  Jersey,  have  many. 
In  the  East  are  the  intruded  sheets  of  Triassic  basaltic  rocks, 
largely  diabases  and  described  below.  They  may  reach  500  feet 
in  thickness,  and  form  many  of  the  most  prominent  landmarks, 
such  as  Cape  Blomidon,  N.  S.;  Mts.  Tom  and  Holyoke,  Mass.; 
East  and  West  Rock,  near  New  Haven,  Conn.,  the  Palisades  on 
the  Hudson,  and  many  dikes  in  the  Richmond,  Va.,  and  Deep 
River,  N.  C.,  coal  fields.  Around  Lake  Superior,  both  in  the  iron 
and  in  the  copper  regions,  are  still  greater  sheets,  for  many 
thousands  of  feet  of  basalt  (diabase)  are  present  on  Keweenaw 
Point.  On  the  north  shore  near  Port  Arthur,  the  head-lands  of 
Thunder  Bay  exhibit  superb  examples.  The  iron-bearing  strata 


76  A  HAND  BOOK  OF  ROCKS. 

are  penetrated  by  innumerable  dikes.  The  greatest  of  all  the  Amer- 
ican basaltic  areas  is,  however,  met  in  the  Snake  River  region  of 
southern  Idaho  and  extends  into  eastern  Oregon  and  Washington. 
Many  thousands  of  square  miles  are  covered  with  the  dark  lava 
and  are  locally  called  the  "  Lava  Beds."  In  Colorado,  as  at  the 
Table  Mountains,  near  Golden  and  Fisher's  Peak,  near  Trinidad, 
there  are  prominent  sheets,  and  the  same  is  true  of  many  other 
points  in  this  State.  In  New  Mexico,  Arizona  and  Texas  they  are 
also  met.  The  volcanoes  of  the  Sandwich  Islands  are  basaltic. 
Basaltic  rocks  with  nephelite  are  scarcely  known  in  the  United 
States.  Some  minor  dikes  in  the  East,  a  volcanic  neck  at  Pilot 
Knob,  near  Austin,  Texas,  dikes  and  sheets  in  Uvalde  Co., 
Texas,  and  a  few  dikes  at  Cripple  Creek,  Colorado,  are  practically 
the  only  localities  yet  identified.  Leucitic  rocks,  more  phono- 
litic  than  basaltic,  are  known  in  the  Leucite  Hills,  Wyo.,  and  in 
Arkansas.  Of  basaltic  affinities  they  occur  in  New  Jersey,  but 
these  and  the  nepheline  rocks  are  of  small  practical  moment, 
although  of  great  scientific  interest. 

Basalts  have  quite  as  great  development  abroad  as  here.  The 
islands  off  the  north  coast  of  Scotland  are  famous  localities,  and 
many  of  the  volcanic  regions  of  the  continent  are  no  less  well 
provided.  The  lavas  of  Etna  are  chiefly  basaltic,  and  those  of 
Vesuvius  are  remarkable  for  their  richness  in  leucite.  In  India 
are  the  great  basalt  fields  of  the  Deccan,  which  are  comparable  in 
extent  with  those  of  the  Snake  River  region  of  the  West. 

THE  DIABASES. 

SiO,  A1,O,  Fe,,O,  FeO  CaO  MgO  Na.O  K,O  Loss.  Sp.  Gr. 

I.   54.52  19.10  2.83  5.89  7.25  3.92  3.73  2.30  0.59  2.7 

2-   53-13  13-74  i-°8  9-10  9-47  8.58  2-3°  I-°3  °-9°  2.96 

3.  49.28  15.92  1.91  10.20  7.44  5.99  3.40  0.72  3.90  2.86 

4.  48.75  I7.I7  0.41  13.62  8.82  3.37  1.63  2.40  ...  2.985 

5.  46.28        12.96        4.67        6.06      10.12        8.71  3.75  3.34         2.921 

6.  45.46        19.94      I5.36          ...          8.32        2.95        2.12        3.21        2.30         2.945 

I.  Diabase  Hills,  Nev.,  Woodward,  40*  Parallel  Surv.,  I.,  Table  opposite  p.  676. 
2.  Penn.  R.  R.  cut,  Jersey  City,  N.  J.,  G.  W.  Hawes,  A.  J.  S.,  Hi.,  IX.,  186.  3. 
Lake  Saltonstall,  Conn.,  Ibid.  4.  Dike  near  Boston,  Mass.,  W.  H.  Hobbs,  Bull. 
Mus.  Comp.  Zool.,  XVI.,  I.  5.  Point  Bonita,  Calif.,  F.  L.  Ransome,  Bull.  Geol. 
Dept.  Univ.  Calif.,  I.,  106.  6.  Dike  at  Palmer  Hill,  Ausable  Forks,  N.  Y.,  J.  F. 
Kemp,  Bull.  107,  U.  S.  G.  S.,  26. 


IGNEOUS  ROCKS.  77 

The  diabases  constitute  a  transitional  group  as  regards  texture 
from  the  felsitic  basalts  to  the  granitoid  gabbros.  The  low  fusing 
points  and  the  great  tendency  to  crystallize,  which  are  possessed 
by  the  basaltic  rocks,  lead  to  the  development  of  rather  finely  yet 
entirely  and  visibly  crystallized  aggregates  of  plagioclase,  augite 
and  often  olivine,  which  in  the  refinements  of  texture  differ  from 
the  true  gabbros.  The  plagioclase  is  in  elongated  and  sharply 
rectangular  rods,  especially  when  viewed  with  the  microscope. 
The  augite  and  olivine,  in  irregular  development,  are  packed  in 
between  these  interlacing  rods.  It  is  evident  that  the  plagioclase, 


FlG.  32.  Diabasic  texture,  drawn  from  a  microscopic  slide  of  a  diabase  from  Pig- 
eon Ft,  Minn.  The  actual  field  was  three  eighths  inch  ( I  cm. ).  The  rods  of  plagio- 
clase are  shown  enclosing  the  augite,  olivine  and  magnetite.  The  shaded  mineral  with 
light  borders  is  augite  ;  the  shaded  mineral  with  heavy  borders  is  olivine.  The  black 
mineral  is  magnetite. 

contrary  to  the  usual  order  of  crystallization,  has  abnormally  fin- 
ished its  period  before  the  ferro-magnesian  silicates  were  much 
advanced,  and  that  the  latter  have  been  forced  to  adapt  themselves 
to  whatever  space  remained.  This  result  has  often  been  reached 
in  dikes  and  in  the  interiors  of  thick  surface  flows  and  intruded 
sheets,  whose  outer  parts  are  characteristic  basalts  or  even  amygda- 
loidal  varieties.  The  diabases  are  thus  a  peculiar  phase  of  the 
granitoid  texture  as  here  defined,  and  their  texture  is  called  the 
diabasic  or  diabasic  granular.  They  differ  from  the  typical  gab- 


78  A  HAND  BOOK  OF  ROCKS, 

bros  in  that  the  feldspars  of  the  latter  are  about  as  broad  as  long, 
and  the  succession  of  generations  in  the  crystallization  of  the  com- 
ponent minerals  of  the  gabbros  has  been  normal.  Yet  intermediate 
textures  are  known  and  gabbros  are  sometimes  described  as  dia- 
basic.  When  in  the  true  diabases  the  augites  become  so  abundant, 
large  and  coarsely  crystalline,  as  to  include  the  rectangular  rods 
of  plagioclase  in  a  matrix  of  pyroxene,  the  texture  is  called  ophitic. 
The  ophitic  texture  is  usually  a  matter  for  the  microscope,  but  the 
diabasic  can  often  be  detected  by  the  eye,  and  the  feldspar  rods 
may,  in  extreme  cases,  be  an  inch  or  more  in  length. 

THE  GABBROS. 


SiO, 

AlaO, 

FetO, 

FeO 

CaO 

MgO 

Na,O 

K2O 

Loss. 

Sp.  Gr. 

I. 

59-55 

25.62 

0-75 

7-73 

tr. 

5.09 

0.96 

0-45 

2.66 

2. 

55-34 

16.37 

0.77 

7-54 

7.51 

5-05 

4.06 

2.03 

0.58 

3- 

54-72 

17.79 

2.08 

6.03 

6.84 

5-85 

3.02 

3-01 

... 

2.928 

4- 

54-47 

26.45 

1.30 

0.67 

10.80 

0.69 

4-37 

0.92 

0-53 

2.72 

5- 

53-43 

28.01 

0-75 

... 

11.24 

0.63 

4-85 

0.96 

tr. 

2.67 

6. 

52.14 

29.17 

3-26 

10.81 

0.76 

3-02 

0.98 

o.S8 

7- 

49-^5 

21.90 

6.60 

4-54 

8.22 

3-03 

3-83 

1.61 

1.92 

8. 

48.02 

17.50 

i.  80 

7.83 

13.16 

IO.2I 

1.48 

tr. 

0.79 

9- 

46.85 

19.72 

3-22 

7-99 

13.10 

7-75 

1.56 

0.09 

0.56 

10. 

46-85 

18.00 

6.16 

8.76 

10.17 

8-43 

2.19 

0.09 

0.30 

3-097 

n. 

45.66 

16.44 

0.66 

13.90 

7.23 

11.57 

2.13 

0.41 

0.07 

I.  Anorthosite,  Chateau  Richer,  Canada,  T.  S.  Hunt,  Geology  of  Canada,  1863. 
2.  Norite,  Cortlandt  Series,  Montrose  Point,  Hudson  River,  Anals.  by  Munn,  for  J.  D. 
Dana,  A.  J.  S.,  Aug.,  1881,  p.  104.  3.  Gabbro,  near  Cornell  Dam,  Croton  River, 
H.  T.  Vulte,  for  J.  F.  Kemp,  unpublished.  4.  Anorthosite,  Summit  of  Mt.  Marcy, 
Adirondacks,  A.  R.  Leeds,  3Oth  Ann.  Rep.  N.  Y.  State  Museum,  reprint,  p.  14,  1876. 
5.  Anorthosite,  Nain,  Labrador,  A.  Wichmann,  Z.  d.  d.  g.  Ges.,  1884.  6.  Gabbro, 
Iron  Mtn.,  Wyo.,  4Oth  Parallel  Surv.,  II.,  14.  7.  Gabbro,  near  Duluth,  Minn., 
Streng,  Neues  Jahrb.,  1876,  117.  8.  Gabbro-diorite,  Baltimore,  Md.,  average  of 
seventeen  samples,  Mackay  for  G.  H.  Williams,  U.  S.  G.  S.,  Bull.  XXVIII.,  39.  9. 
Gabbro,  Baltimore  average  of  twenty-three  samples,  ibid.  10.  Gabbro,  Southwest 
Adirondacks,  C.  H.  Smyth,  Jr.,  A.  J.  S.,  July,  1894,  61.  n.  Gabbro,  Northwest 
Minn.,  W.  S.  Bayley,  Anals.  by  Stokes,  Jour.  Geol.,  I.,  712. 

Comments  on  the  Analyses.  —  The  range  in  composition  pre- 
sented by  the  gabbros  is  in  many  respects  the  same  as  that  of  the 
basalts,  just  as  we  would  naturally  expect;  but  there  is  one 
analysis  (No.  i)  which  goes  higher  in  silica.  The  rock  is,  how- 
ever, a  variant  from  the  typical  gabbros,  as  may  be  seen  from  the 
lack  of  iron  and  magnesia,  leaving  little  else  than  the  necessary 
elements  for  plagioclase.  The  same  is  true  of  Nos.  4  and  5.  The 


IGNEOUS  ROCKS. 


79 


other  analyses,  however,  exhibit  very  characteristic  percentages, 
and  upon  studying  them  with  care  the  reader  will  see  that  the 
aggregate  of  plagioclase,  augite  and  often  olivine,  which  is  what 
constitutes  the  typical  gabbro,  must  result  from  their  crystallization. 
The  percentages  in  silica  range  from  55  to  45.  The  alumina  is  in 
general  quite  high.  In  No.  6  it  reaches  a  maximum  for  all  the 
igneous  rocks  given.  Except  in  I,  4,  5  and  6,  the  bases  iron, 
magnesia,  lime  and  soda  are  quite  high.  Potash  naturally  is  low, 
since  orthoclase  is  usually  present  in  small  amount. 


FlGS.  33  AND  34.  Diagrams  illustrating  the  chemical  composition  of  the  gabbros 
which  are  given  in  the  above  analyses.  Fig.  33  is  based  on  molecular  ratios  ;  Fig.  34 
on  percentages. 

Figs.  33  and  34  are  very  like  those  of  the  basalts,  Figs.  28  and 
29.  Lime,  however,  is  relatively  much  greater  than  magnesia, 
owing  in  part  to  the  fact  that  analyses  I,  4,  5  and  6  are  included  in 
the  general  average.  The  FeO  is  greater  than  the  Fe2O3,  just  re- 
versing the  relations  shown  in  the  diagram  of  the  basalts,  and 
doubtless  due  to  these  particular  selections  of  analyses.  There 


80  A  HAND  BOOK  OF  ROCKS. 

seems  to  be  no  fundamental  reason  why  the  basalts  should  differ 
from  the  gabbros  in  these  respects. 

Mineralogical  Composition,  Varieties.  —  The  name  gabbro  is  of 
Italian  origin,  and  has  been  applied  in  recent  years,  and  with 
growing  favor  to  the  great  group  of  granitoid  rocks  which  consists, 
in  the  typical  cases,  of  plagioclase  and  pyroxene.  The  diabases,  as 
was  explained  above,  are  texturally  and  mineralogically  transitions 
from  the  basalts  to  the  true  gabbros.  The  so-called  gabbro  group 
is  a  very  large  and  characteristically  variable  one.  Originally  the 
name  gabbro  was  only  applied  to  a  mixture  of  plagioclase  and  the 
variety  of  monoclinic  pyroxene  called  diallage,  that  has  pinacoidal 
as  well  as  prismatic  cleavages,  but  of  late  years  all  granitoid,  plu- 
tonic,  pyroxene-plagioclase  rocks  are  collectively  spoken  of  as  the 
gabbro  group.  In  the  typical  gabbros  the  dark  silicates  predomi- 
nate over  the  light-colored  ones,  but  rocks  are  included  in  the 
general  group,  to  which  this  restriction  does  not  apply.  In  this 
particular  the  facts  of  field  occurrence  and  natural  relationship  have 
broken  down  the  sharpness  of  mineralogical  definitions.  At  the 
acidic  extreme  we  have  in  Canada  and  the  Adirondacks  enormous 
masses  of  rock  that  are  practically  pure,  coarsely  crystalline  labra- 
dorite.  Pyroxene  is  the  dark  silicate  when  any  is  present,  but 
often  it  is  insignificant.  These  pure  feldspar  rocks  are  best  called 
anorthosites,  from  the  French  word  for  triclinic  feldspar,  but  the 
word  is  not  to  be  confused  with  anorthite,  the  lime  feldspar,  with 
which  it  has  no  special  connection.  An  old  and  obsolete  synonym 
of  anorthosite  is  labradorite-rock,  of  interest  because  widely  used 
in  early  reports  on  the  Adirondacks.  As  monoclinic  pyroxene  in- 
creases the  rocks  pass  into  gabbros  proper.  More  or  less  biotite 
and  hornblende  may  also  be  present.  If  the  pyroxene  is  ortho- 
rhombic  we  call  the  rock  norite.  Varieties  with  olivine  are  frequent, 
giving  olivine-gabbro  and  olivine-norite.  Gabbros  and  norites 
are  not  readily  distinguished  without  the  microscope,  unless  the 
bronzy  appearance  of  hypersthene  can  be  recognized.  In  the  former 
case,  gabbro  is  a  good  collective  term.  Norites  were  formerly 
called  hypersthene  rock,  or  hypersthene-fels,  both  of  which  are 
undesirable  rock  names.  Gabbro  intrusions  of  not  too  great  extent 
for  careful  study  have  been  observed  to  grow  more  basic  toward 
the  outer  margins. 


IGNEOUS  ROCKS.  81 


THE  PYROXENITES  AND  PERIDOTITES. 

Pyrox.    SiO,  AltO,  Fe,O,  FeO  CaO  MgO  N»,O  K,O        Loss.     Sp.  Or. 

1.  55.14  0.25  3.48  4.73         8.39  26.66  0.30         

2.  53.98  1.32  I.4I  3.90  15.47  22.59  ...            ...          0.83        3.301 

3.  44.01  11.76  15.01  ...           4.06  25.25  

Peridotite. 

4.  47.41  6.39  7.06  4.80  14.32  15.34  0.69  1.40        2.10        3.30 
5-            46.03  9.27  2.72  9.94          3.53  25.04  1.48  0.87        0.64        3.228 

6.         41.00       7.58         ...         5.99      10.08       23.59      0.52        ...         4.73       2.989 

7-  36.80        4.16  ...  8.33          8.63         25.98        0.17         2.48        0.51 

8.  33.84      5.88       7.04      5.16       9.52      22.96      0.33      2.04      7.50 

9.  29.81         2.01          5.16        4.35          7.69        32.41         O.I  I         0.20        8.92         2.78 

I.  Pyroxenite,  var.  Websterite,  Webster,  N.  C.,  E.  A.  Schneider  for  Geo.  H.  Wil- 
liams, Amer.  Geol.,  July,  1890,  p.  41.  2.  Pyroxenite,  Baltimore,  Md.,  T.  M.  Chatard 
for  G.  H.  Williams,  ibid.  3.  Pyroxenite,  Meadow  Creek,  Mont.,  Geo.  P.  Merrill, 
Proc.  U.  S.  Natl.  Mus.,  XVII.,  658.  4.  Peridotite,  Cortland  Series,  Montrose  Pt, 
N.  Y.,  Emerson  for  G.  H.  Williams,  A.  J.  S.,  Jan.,  1886,  40.  5.  Peridotite,  Custer 
Co.,  Colo.,  L.  G.  EakinsforW.  Cross,  Proc.  Colo.  Sci.  Soc.,  1887,  245.  6.  Peridotite, 
Baltimore,  Md.,  L.  Mackay  for  G.  H.  Williams,  Amer.  Geol.,  July,  1890,  39.  7. 
Peridotite,  Dewitt,  N.  Y.,  H.  S.  Stokes  for  Darton  and  Kemp,  Amer.  Jour.  Sci.,  June, 
1895,  456.  8.  Mica  Peridotite,  Crittenden  Co.,  Ky.,  W.  F.  Hillebrand  for  J.  S.  Dil- 
ler,  A.  J.  S.,  Oct.,  1892,  288.  9.  Peridotite,  Elliott  Co.,  Ky.,  J.  S.  Diller,  Bull.  38, 
U.  S.  G.  S.,  p.  24. 

Comments  on  the  Analyses. — As  compared  with  the  gabbros  the 
pyroxenites  are  characterized  by  the  falling  off  in  A13O3,  due  to 
the  disappearance  of  feldspar,  and  by  the  increase  in  CaO  and 
MgO  necessitated  by  the  predominance  of  the  pyroxenes.  The 
peridotites  reach  a  lower  percentage  of  silica  than  any  other 
igneous  rocks  so  far  cited,  but  if  this  is  accompanied  by  high  H2O, 
allowance  must  be  made  for  the  relative  decrease  of  the  original 
SiO2  because  the  rock  has  obviously  changed  to  serpentine.  The 
great  percentages  of  MgO  are  very  notable,  and  are  due  to  the 
presence  of  much  olivine,  magnesian  pyroxene  and,  in  instances, 
biotite.  Chromic  oxide  is  also  always  present  in  small  amounts, 
and  oxides  of  nickel  and  cobalt  are  usually  in  perceptible  quantity. 

The  diagrams  illustrating  the  pyroxenites  and  peridotites  are  of 
much  interest  when  compared  with  those  of  the  gabbros.  The 
intercepts  for  silica  have  drawn  in ;  those  for  alumina  and  the 
alkalies  have  almost  disappeared.  The  iron  and  lime  have  not 
changed  very  greatly,  but  the  magnesia  has  expanded  enormously, 
and  has  afforded  a  very  significant  and  interesting  set  of  figures. 
6 


82 


A  HAND  BOOK  OF  ROCKS. 


FIGS.  35  AND  36.  Diagrams  illustrating  the  chemical  composition  of  the  pyroxenites, 
which  are  given  in  the  above  analyses.  Fig.  35  is  based  on  molecular  ratios ;  Fig.  36 
on  percentages. 


FIGS.  37  AND  38.  Diagrams  illustrating  the  chemical  composition  of  the  peridotites, 
which  are  given  in  the  above  analyses.  Fig.  37  is  based  on  molecular  ratios  ;  Fig.  38 
on  percentages. 


IGNEOUS  ROCKS.  83 

The  resemblance  to  the  figures  of  the  limburgites  given  above 
(Figs.  30  and  31)  is  close. 

Mneralogical  Composition,  Varieties. — The  gabbros  pass  in- 
sensibly, by  the  decrease  of  plagioclase,  into  the  pyroxenites  and 
peridotites,  and  in  any  great  gabbro  area  all  these  are  usually 
present,  but  they  may  occur  also  as  independent  masses.  The 
pyroxenites  are  practically  pyroxene,  with  little  if  any  other  min- 
erals. There  is  some  variety,  according  as  the  rock  contains  one 
or  several  of  the  following :  enstatite,  bronzite,  hypersthene,  dial- 
lage  or  augite ;  but  with  the  unassisted  eye,  it  is  seldom  that  one 
can  be  sure  of  these  distinctions,  except  as  the  orthorhombic 
pyroxenes  exhibit  a  bronze  luster.  Hornblende,  magnetite  and 
pyrrhotite  may  also  be  present.  With  the  accession  of  olivine, 
peridotite  results,  so  named  from  the  French  word,  peridot,  for 
olivine,  and  a  number  of  varieties  have  been  made  according  as  the 
olivine  is  associated  with  one  or  more  of  the  minerals  cited  for 
pyroxenites.  The  list  is  given  under  Peridotite  in  the  Glossary. 
The  distinctions  are  however  hardly  possible  without  microscopic 
aid.  As  the  extreme  of  peridotites  we  have  a  nearly  pure  olivine 
rock,  called  dunite,  important  in  North  Carolina.  Much  magnetite 
may  be  associated  with  peridotite ;  indeed  at  Cumberland  Hill,  R. 
I.,  there  is  enough  to  almost  make  the  rock  an  ore.  Chromite, 
too,  is  a  frequent  associate.  As  peridotites  shade  into  a  porphyritic 
texture,  especially  in  dikes,  they  have  been  called  picrites,  and 
even  additional  varieties,  such  as  kimberlite,  have  been  made. 
Black  hornblende,  which  is  brown  in  thin  sections,  is  frequent  in 
both  pyroxenites  and  peridotites,  and  may  even  form  a  rock  itself, 
hornblendite.  Dark  brown  biotite  is  also  often  present  in  con- 
siderable amount. 

Some  writers  have  regarded  the  pyroxenites  and  peridotites  as 
of  doubtful  igneous  origin  and  have  placed  them  with  metamorphic 
rocks,  but  from  their  frequent  association  with  gabbro,  and  from 
their  independent  occurrence  in  dikes,  there  is  no  good  reason  to 
doubt  their  true,  igneous  nature. 

A  very  rare  granitoid  rock,  consisting  of  plagioclase,  nepheline  and 
ferro-magnesian  silicates  has  been  called  theralite  from  the  Greek 
verb  to  seek  eagerly,  because  its  discovery  was  anticipated  by  H. 
Rosenbusch  before  it  was  actually  found  by  J.  E.  Wolff  in  the  Crazy 


84  A  HAND  BOOK  OF  ROCKS. 

Mountains,  Montana.  It  is  an  extremely  rare  combination  of  min- 
erals, but  of  special  scientific  interest  because  it  corresponds  among 
the  granitoid  rocks  to  the  tephrites  and  basanites  of  the  porphyritic. 

Alteration,  Metamorphism.  —  The  gabbros  alter  chiefly  by  the 
formation  of  serpentine  and  chlorite  from  the  dark  silicates.  The 
pyroxenites  and  peridotites  change  readily  into  serpentine,  often 
with  an  intermediate  stage  as  hornblende-schist.  Under  dynamic 
stresses,  especially  shearing,  anorthosites  and  gabbros  pass  into 
gneissoid  types,  and  in  the  process  much  garnet  may  be  developed. 
This  is  especially  true  in  the  Adirondacks.  The  larger  feldspars 
may  be  left  in  the  gneisses  as  "  eyes,"  or,  to  adopt  the  German 
term,  as  "Augen,"  affording  Augen-gneiss,  i.  e.,  gneisses  with 
comparatively  large  lenticular  feldspars.  Much  hornblende,  espe- 
cially in  true  gabbros,  is  often  developed  in  the  process.  The 
basic  members,  the  pyroxenites  and  peridotites  develop  into 
amphibolites  or  hornblende- schists,  which  latter  often  furnish  very 
puzzling  geological  problems. 

Distribution.  —  The  anorthosites  occur  in  several  Canadian 
areas,  as  at  the  headwaters  of  the  Saguenay  River,  and  again  north 
of  Montreal ;  in  the  higher  peaks  of  the  Adirondacks  and  some  of 
their  outliers  such  as  Mt.  Marcy  and  its  neighbors ;  and  to  the 
northeast  of  Laramie,  Wyo.,  in  the  Laramie  range.  Gabbros  are 
also  present  in  vast  quantity  in  the  Adirondacks  and  are  likewise 
well  known  in  the  White  Mountains,  in  the  famous  Cortlandt 
series,  near  Peekskill,  on  the  Hudson,  and  in  the  vicinity  of  Balti- 
more. Around  Lake  Superior  gabbros  are  of  great  importance. 
The  basal  members  of  the  Keweenawan  system  and  other  older 
intrusions  are  largely  formed  of  them.  Fine  specimens  can  be 
had  at  Duluth.  Gabbro  is  a  characteristic  wall  rock  of  titaniferous 
magnetite.  Pyroxenites  occur  as  subordinate  members  of  the 
gabbro  areas,  especially  near  Baltimore.  Peridotites  are  in  the 
same  relations  in  the  Cortlandt  series,  in  the  Baltimore  area  and  in 
North  Carolina.  They  are  also  known  on  Little  Deer  Island, 
Me.;  at  Cumberland  Hill,  R.  I.;  in  dikes  near  Syracuse,  N.  Y.; 
at  Presqu'  Isle,  near  Marquette,  Mich.;  in  Kentucky;  in  Cali- 
fornia and  elsewhere  in  the  West.  When  outlying  dikes  are  met, 
far  from  any  visible,  parent  mass  of  igneous  rocks  and  in  sedi- 
mentary walls,  they  are  very  frequently  peridotite. 


IGNEOUS  ROCKS.  85 

Abroad,  anorthosites  and  gabbros  are  abundant  in  the  Scandi- 
navian peninsula,  whose  geology  is  in  many  respects  like  that  of 
Canada  and  the  Adirondacks.  In  the  north  of  Scotland  gabbros 
are  of  especial  interest  because  they  have  been  shown  by  Judd  to 
be  the  deep-seated  representatives  of  the  surface  basalts.  On  the 
continent  they  are  important  rocks  in  many  localities.  The  same 
is  true  of  Australia  and  such  other  parts  of  the  world  as  have  been 
studied.  Of  especial  interest  are  the  peridotite  dikes  in  South 
Africa  that  have  proved  to  be  the  matrix  of  the  diamond. 

ULTRA-BASIC  IGNEOUS  ROCKS.     METEORITES. 

A  few  ultra-basic  igneous  rocks  are  known  in  which  the  silica 
decreases  almost  to  nil,  and  in  which  the  bases,  especially  iron,  are 
correspondingly  high.  They  are  in  general  rather  to  be  con- 
sidered as  basic  segregations  in  a  cooling  and  crystallizing  magma 
than  as  individual  intrusions.  The  Cumberland  Hill,  R.  I.,  so- 
called  peridotite,  cited  above,  has  very  little  silica.  Titaniferous 
ores  have  almost  none,  but  they  are  often  exceptionally  rich  in 
alumina.  In  a  few  cases  metallic  iron  has  been  detected  in  basic 
igneous  rocks,  suggesting  analogies  with  meteorities. 

Meteorites  are  rare  and  only  of  scientific  interest,  but  it  is 
extremely  suggestive  that  such  silicates  as  are  met  in  them  are 
chiefly  olivine  and  enstatite,  minerals  rather  characteristic  of  very 
basic  rocks.  The  commoner  meteorites  are  an  alloy  of  metallic 
iron  and  nickel,  but  some  rare  sulphides  are  occasionally  present. 

As  filling  out  the  theoretical  series  we  cannot  bar  out  water  and 
ice.  There  is  no  reason  why  they  should  not  be  considered  igneous 
rocks  of  extremely  low  fusing  point,  but  they  are  so  familiar  that 
a  simple  reference  to  them  is  sufficient 


86  A  HAND  BOOK  OF  ROCKS. 

The  Basalt-Peridotite  Table.  —  The  same  general  remarks  which 
are  given  on  p.  38,  regarding  the  recasting  of  the  analyses,  apply 
also  to  this  table,  but  some  additional  difficulties  arise.  Thus  in 
No.  73.3,  the  limburgite,  as  described  by  the  author,  we  have  a 
rock  with  a  glassy  groundmass,  in  which  pyroxene,  olivine, 
magnetite,  ilmenite  and  apatite  are  contained.  In  recasting  we 
are  practically  forced  to  assume  leucite,  analcite  and  nephelite, 
and  then  to  refer  the  substance  of  these  to  the  glassy  groundmass. 
In  this  case  the  recasting  is  only  a  remote  approximation.  By 
study  of  the  table  we  see  that  quartz  appears  sometimes  in  the 
medium  or  basic  rocks.  Orthoclase  also  maintains  a  subordinate 
place.  Plagioclase,  except  in  the  limburgite,  pyroxenite  and 
peridotite,  is,  as  a  rule,  the  most  abundant  mineral  and  in  it  the 
anorthite  molecule  predominates.  Herein  lies  the  marked  con- 
trast with  the  dacite-diorite  series.  In  the  anorthosite  No.  78.4, 
the  plagioclase  reaches  its  culmination,  constituting  87. 1  per  cent. 
of  the  volume  of  the  rock,  and  with  the  orthoclase  and  quartz, 
94.9  or  say  95  per  cent.  The  anorthosites  thus  furnish  a  very 
peculiar  magma,  of  unusual  scientific  interest.  If  the  loss  is 
counted  as  a  light-colored  mineral  we  obtain  our  maximum  of 
96.2,  leaving  only  3.8  for  the  pyroxene  and  magnetite.  No.  78.7 
is  next  in  percentage  of  light-colored  minerals  (86. 5  per  cent,  by 
volume)  and  is  the  famous  gabbro  of  Duluth,  Minn.,  with  its 
unusual  amounts  of  orthoclase.  From  these  values  we  drop  to  a 
minimum  for  the  basalts  of  72.2  and  for  the  gabbros  of  66.1.  In 
the  well-known  diabase  of  the  Palisades,  N.  J.,  No.  76. 2,  there  is 
56.9.  We  see  thus  that  the  light-colored  minerals  preponderate  in 
these  rocks,  but  as  the  feldspar  is  sometimes  a  green  or  red,  the 
rocks  are  darker  than  we  might  expect  from  the  proportions  given. 
In  the  well-crystallized,  feldspar-free  rocks  relations  change.  The 
one  pyroxenite  cited  has  93.3  per  cent,  of  dark  minerals.  The 
peridotites  are  81.6  and  98  respectively.  The  last  named  rock,  a 
curious  peridotite  dike  from  Kentucky  has  only  apatite  which  may 
be  considered  a  light-colored  mineral. 

Recalling  the  remarks  made  upon  the  preceding  tables  as  well 
as  the  one  just  discussed  we  are  impressed  with  the  very  general 
predominance  of  light-colored  minerals  in  the  igneous  rocks,  a 
characteristic  which  only  changes  in  the  decidedly  basic. 


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CHAPTER   VI. 
REMARKS  IN  REVIEW  OF  THE  IGNEOUS  ROCKS. 

Chemical  Composition.  —  Igneous  magmas  vary  from  about  80 
per  cent,  silica  as  a  maximum  to  practically  none  as  a  minimum, 
but  important  rocks  rarely  drop  below  40  per  cent.  Alumina  is 
highest  in  the  anorthosites  or  feldspathic  gabbros,  where  it  may 
exceed  25  per  cent.  It  is  lowest  in  the  pyroxenites  and  may  be 
less  than  i  per  cent.  The  oxides  of  iron  are  almost  lacking  in  the 
highly  siliceous,  but  may  reach  beyond  20  per  cent,  in  the  basaltic 


RHYOLITES  AND  GRANITES 


TRACHYTES-SYENITES  RARE  BASIC 

PHONOLITES   NEPH.  SYEN'T'S          ORTHOCLASE 
ROCKS 


DACITES     ANDESITES  BASALT  GROUP 

Q'TZ   DIORITES      DIORITES      GABBROS      PYROXENITES   PERIDOTITES   ORES 


FIGS.  39  AND  40.  —  Diagram  intended  to  illustrate  graphically  the  mineralogical 
composition  of  the  igneous  rocks.  The  numbers  indicate  percentages  in  silica.  The 
upper  diagram,  Fig.  39,  includes  the  ortboclase  rocks,  the  lower,  Fig.  40,  the  plagio- 
clase  and  non-feldspathic  ones.  Q  is  quartz  ;  O,  orthoclase  ;  NL,  nephelite  and  leu- 
cite  ;  P,  plagioclase ;  M,  muscovite ;  B,  biotite ;  H,  hornblende ;  A,  augite ;  Ol, 
olivine ;  the  black  area,  magnetite,  pyrrhotite,  and  other  metallic  minerals. 

rocks,  and  with  TiO2  may  be  nearly  100  per  cent,  in  some  igneous 
iron  ores.  Lime  attains  its  maximum  of  12-15  per  cent,  in  the 
gabbros  and  pyroxenites,  while  magnesia  in  the  pyroxenites  and 
peridotites  may  even  surpass  25  per  cent.  Potash  is  most  abun- 
dant in  the  orthoclase  and  leucite  rocks  ;  soda  in  those  with  nephe- 

88 


IGNEOUS  ROCKS.  89 

lite.  Combined  alkalies  may  reach  1 5  per  cent,  in  the  phonolites 
and  nephelite  syenites.  In  general  they  are,  however,  about  4-10, 
and  may  practically  fail.  Water  in  quantities  over  i  per  cent,  as  a 
rule  is  an  indication  of  decay,  but  in  the  pitchstones  this  is  not 
positively  true,  for  the  water  reaches  close  to  10  per  cent,  and  the 
rocks  are  apparently  unaltered. 

Texture.  —  All  of  the  typical  textures  are  easily  recognized  in 
characteristic  development,  but  the  glassy  shade  insensibly  into  the 
felsitic,  the  felsitic  into  the  porphyritic,  and  the  porphyritic  into  the 
granitoid.  There  are,  therefore,  intermediate  forms  that  are  diffi- 
cult to  classify.  Yet,  on  the  whole,  the  four  textures  are  the  most 
satisfactory  basis  for  classification,  and  as  a  guide,  in  accordance 
with  which  to  study.  Chemical  composition  being  the  same,  texture 
is  a  result  of  the  physical  conditions  surrounding  the  magma  at  the 
time  of  crystallization  and  of  the  presence  of  mineralizers. 

Mineralogy. — The  above  diagrams,  with  a  reasonable  approxi- 
mation to  the  truth,  illustrate  the  quantitative  mineralogy  of  the 
igneous  rocks.  A  section  cut  through  the  charts  at  any  one  point 
expresses  the  relative  amounts  as  well  as  kinds  of  the  several  min- 
erals in  the  rocks  whose  names  are  along  the  top  lines,  and  whose 
percentages  in  silica  are  approximately  shown.  No  mention  is 
made  of  texture.  In  the  orthoclase  rocks  quartz  disappears  at 
about  65^)  SiO2,  while  orthoclase  continues  to  the  end;  plagio- 
clase  in  small  amount  is  quite  constantly  present  throughout  the 
series.  Nephelite  and  leucite  come  in  as  indicated.  Muscovite 
appears  only  in  the  more  acidic  granites.  Biotite  and  hornblende 
vary  in  relative  quantity,  but  toward  the  basic  end  both  yield  to 
augite.  The  rocks  at  the  basic  end  are  chiefly  those  recently  dis- 
covered by  Weed  and  Pirsson  in  Montana,  by  Iddings  in  the 
Yellowstone  Park  and  by  Lawson  in  the  Rainy  Lake  region.  In 
the  plagioclase  and  non-feldspathic  rocks  quartz  and  orthoclase 
soon  run  out,  so  far  as  any  notable  or  regular  amount  is  concerned. 
Plagioclase  holds  along  to  about  45^  SiO2,  and  at  about  55^  SiO2 
may  in  the  anorthosites  be  the  only  mineral  present.  Biotite  and 
hornblende  are  present  all  the  way  through,  but  toward  the  basic 
end  they  tend  to  yield  in  importance  to  augite,  which  latter  in  some 
pyroxenites,  at  about  49^  SiO2,  may  be  the  only  silicate  present. 
Olivine  begins  to  appear  at  55/6  and  steadily  increases  with  occa- 


90  A  HAND  BOOK  OF  ROCKS. 

sional  lapses  almost  to  the  end,  where  it  may  be  the  chief  mineral. 
The  ores,  as  the  extreme  case,  and  without  regard  to  silica,  increase 
so  as  to  be  the  only  minerals  in  the  rock,  forming  thus  the  theo- 
retical, basic  limit.  The  diagrams  also  emphasize  the  fact  that 
igneous  rocks  shade  into  one  another  by  imperceptible  gradations, 
and  this  is  true  of  the  orthoclase  and  plagioclase  groups  them- 
selves, although  not  suggested  by  the  separation  of  the  two  in  the 
drawings.  The  continuation  of  the  orthoclase  series  to  a  basic 
extreme  is  a  fact  that  we  have  only  appreciated  in  very  recent  years. 

A  careful  scrutiny  of  analyses  and  mineralogical  composition 
leads  to  the  conclusion  that  practically  the  same  magma  may, 
under  different  physical  conditions  of  crystallization,  afford  miner- 
alogical aggregates  that  vary  considerably  in  the  proportions  of  the 
several  minerals  —  now  yielding  more  hornblende,  again  more 
augite,  and  even  affording  quartz  in  a  basalt.  Hence,  analyses  in 
different  groups  overlap  more  or  less,  and  the  difficulty  of  drawing 
sharp  lines  of  distinction  is  increased.  Yet,  allowing  for  this  vari- 
ation chemical  composition  determines  the  resulting  mineralogical 
aggregate  and  is  fairly  characteristic. 

Determination  of  Igneous  Rocks.  —  In  determining  an  igneous 
rock,  the  texture  should  first  be  regarded,  next  the  feldspars.  If 
orthoclase  prevails,  the  presence  or  absence  of  quartz  establishes 
the  rock.  If  plagioclase  prevails,  we  look  for  biotite,  hornblende, 
pyroxene  and  olivine.  If  no  feldspar  is  present,  we  look  for  the 
presence  or  absence  of  olivine.  On  this  basis  the  table  on  page  23 
is  to  be  used  —  there  are,  however,  many  finely  crystalline  rocks 
which  elude  the  power  of  the  unassisted  eye.  If  of  light  shades, 
they  can  generally  be  referred  with  reasonable  correctness  to  the 
rhyolites,  trachytes,  felsites  or  andesites.  If  dark,  they  are  prob- 
ably basaltic  in  their  nature,  and  the  name  "  trap  "  is  a  very  useful 
and  sufficiently  non-committal  term.  Care  should  be  exercised 
not  to  confuse  the  porphyritic  rocks  having  angular  phenocrysts 
with  the  amygdaloids,  or  the  rocks  whose  almond-shaped  cavities, 
produced  by  expanding  steam,  have  been  filled  by  later  introduced 
calcite,  or  quartz,  or  other  secondary  minerals.  While  books  are 
of  great  assistance,  really  the  only  way  to  become  properly  familiar 
with  rocks  is  to  use  the  books  in  connection  with  correctly  labeled 
and  sufficiently  complete  study  collections. 


IGNEOUS  ROCKS.  91 

Field  Observations.  —  A  rock  is  not  to  be  considered  by  any  in- 
telligent observer  as  a  dead,  inert  mass  in  nature,  but  as  an  im- 
portant participant  in  the  ceaseless  round  of  changes  which  confront 
us  on  every  side.  Familiarity  with  specimens  and  varieties  in 
collections  ought  always  to  be  followed  by  observation  in  the  field. 
We  have  all  grown  to  believe  that  in  limited  areas  igneous  rocks, 
however  varied  they  may  be,  are  yet  intimately  related  in  their 
origin,  or  are  bound  together  by  ties  of  kinship,  "  consanguinity," 
as  Iddings  has  called  it.  Some  regions  like  eastern  Montana  and 
the  Black  Hills  have  especial  richness  of  high  soda  or  potash 
magmas,  giving  rise  to  nephelite  and  leucite  rocks,  and  sodalite 
syenites ;  Colorado,  Utah  and  New  Mexico  have  wonderful  and 
enormous  laccolites  of  andesites  (porphy rites).  The  Pacific  coast 
in  South  America  has  andesites  in  vast  extent  from  active  vol- 
canoes, and  in  North  America  from  extinct  cones.  Idaho,  Ore- 
gon and  Washington  are  marked  by  basalts.  The  Atlantic  coast 
region  has  a  long  series  of  very  ancient  volcanoes,  that  preceded 
the  early  fossiliferous  strata  from  Newfoundland  to  North  Carolina 
and  that  yielded  nearly  the  entire  series  of  the  volcanic  rocks.  In 
the  Adirondacks,  on  the  Hudson  near  Peekskill,  near  Baltimore, 
and  around  Lake  Superior  we  find  the  members  of  the  gabbro 
family;  while  near  tidewater  along  the  Atlantic  seaboard  we  have 
granites,  almost  all  with  biotite.  Such  facts  as  these  suggested 
the  creation  of  the  term  "  petrographic  provinces,"  to  J.  W.  Judd, 
in  the  endeavor  to  suggest  these  kinships  of  magmas  in  certain 
limited  districts.  There  are  many  others  even  in  North  America 
that  could  be  cited,  but  the  above  will  suffice  to  remind  the  reader 
that  these  broader  relationships  should  be  always  before  him  while 
extending  his  acquaintance  with  rocks  as  they  occur  in  the  natural 
world  about  him. 


CHAPTER   VII. 

THE  AQUEOUS  AND  EOLIAN  ROCKS.     INTRODUCTION.     THE  BREC- 
CIAS AND  MECHANICAL  SEDIMENTS  NOT  LIMESTONES. 

The  members  of  this,  the  second  grand  division,  are  much 
simpler,  and,  as  a  general  thing,  much  easier  to  identify  and  to 
understand  than  are  the  igneous.  No  single  term  is  comprehensive 
enough  to  include  them  all,  and  even  the  double  one  selected 
above  in  the  endeavor  to  embrace  as  many  as  possible,  and  to  avoid 
the  multiplication  of  grand  divisions,  still  falls  short  of  including 
several.  Nevertheless,  those  not  embraced  (the  breccias)  are  of 
limited  distribution,  and,  for  many  reasons,  go  best  with  the  other 
fragmental  rocks,  even  if,  strictly  speaking,  they  are  neither  aqueous 
nor  eolian  in  origin.  Sedimentary  is  the  most  useful  term,  and  is 
universally  applied  as  a  partial  synonym  of  the  above,  for  it  fairly 
includes  the  most  important  members  of  the  series,  but  the  rocks 
deposited  from  solution  and  the  eolian  rocks  can  hardly  be  under- 
stood by  it 

The  rocks  will  be  taken  up  under  the  following  groups  : 
I.  Breccias  and  Mechanical  Sediments,  not  Limestones. 
II.  Limestones. 

III.  Organic  Remains,  not  Limestones. 

IV.  Precipitates  from  Solution. 

The  limestones  are  reserved  for  a  special  group,  although  they  be- 
long in  instances  to  each  of  the  other  three.  They  form,  however, 
such  an  important  series  in  their  scientific  and  practical  relations,  that 
it  is  in  many  respects  advantageous  to  take  them  all  up  together. 

I.    BRECCIAS  AND  MECHANICAL  SEDIMENTS,  NOT  LIMESTONES. 

Group  I.  is  described  in  order  from  coarse  to  fine  according  to 
the  following  series,  minor  varieties  not  cited  in  the  table  being 
mentioned  in  the  text  under  their  nearest  relatives. 

COARSB  TO  FINK. 


BRECCIA. 

GRAVEL  AND 
CONGLOMERATE. 

SAND  AND 
SANDSTONE. 

ARGILLACEOUS 
SANDSTONE. 
CALCAREOUS 
SANDSTONE. 

SILT  AND 
SHALE. 
CALCAREOUS 
SHALE. 

CLAY. 

MARL. 

92 


THE  AQUEOUS  AND  EOLIAN  ROCKS.  93 

BRECCIAS. 

The  word  breccia  is  of  Italian  origin  and  is  used  to  describe  ag- 
gregates of  angular  fragments  cemented  together  into  a  coherent 
mass.  The  breccias  cannot  all  be  properly  considered  to  be  either 
aqueous  or  eolian,  and  some  have  already  been  referred  to  under 
the  fragmental  igneous  rocks.  Oftentimes  they  resemble  con- 
glomerates, but,  unless  formed  of  fragments  of  some  soluble  rock, 
whose  edges  have  become  rounded  by  solution,  there  is  no  diffi- 
culty in  distinguishing  them.  Breccias,  as  regards  their  angular 
fragments  and  interstitial  filling,  may  be  of  the  same  materials  or 
of  different  ones.  We  may  distinguish  Friction  breccias  (Fault 
breccias),  Talus  breccias,  and,  for  the  sake  of  completeness,  may 
also  mention  here  Eruptive  breccias. 

Friction  breccias  are  caused  during  earth-movements  by  the 
rubbing  of  the  walls  of  a  fault  on  each  other,  and  by  the  conse- 
quent crushing  of  the  rock.  The  crushed  material  of  finest  grade 
fills  in  the  interstices  between  the  coarser  angular  fragments,  and 
all  the  aggregate  is  soon  cemented  together  by  circulating,  mineral 
waters.  Such  breccias  occur  in  all  rocks  and  are  a  frequent  source 
of  ores,  which  are  introduced  into  the  interstices  by  infiltrating 
solutions.  Quartz  and  calcite  are  the  commonest  cements. 

Talus  breccias  are  formed  by  the  angular  debris  that  falls  at  the 
foot  of  cliffs  and  that  becomes  cemented  together  by  circulating 
waters,  chiefly  those  charged  with  lime. 

Eruptive  breccias  may  be  produced  either  by  the  consolidation  of 
coarse  and  fine,  fragmental  ejectments  like  tuffs,  or  by  an  erupting 
sheet  or  dike  that  gathers  in  from  the  wall  rock  sufficient  fragments 
as  inclusions  to  make  up  the  greater  part  of  its  substance.  These 
are  finally  cemented  together  by  the  igneous  rock  itself  and  afford 
curious  and  interesting  aggregates,  oftentimes  representing  all  the 
rocks  through  which  the  dike  has  forced  its  way  to  the  surface. 
A  crust  may  also  chill  on  a  lava  stream,  and  when  an  added  im- 
pulse starts  anew  the  flowing,  the  crust  may  be  shattered  into  an 
eruptive  breccia  of  a  still  different  type. 

We  often  speak  of  breccias  as  "brecciated  limestone,"  "brecciated 
gneiss,"  or  some  other  rock,  thus  making  prominent  the  character 
of  the  original.  When  the  fragments  and  the  cement  are  con- 


94  A   HAND  BOOK  OF  ROCKS. 

trasted  in  color,  very  beautiful  ornamental  stones  result,  which 
may  be  susceptible  of  a  high  polish. 

A  moment's  consideration  of  the  above  methods  of  origin  will 
convince  the  reader  that  breccias,  except  as  formed  of  loose,  vol- 
canic ejectamenta,  are  of  very  limited  occurrence.  Although  deeply 
buried  rocks  that  share  in  profound  earth  movements  often  suffer 
crushing  and  brecciation  on  a  large  scale,  the  effects  are  chiefly 
detected  by  microscopical  study. 

GENERALITIES  REGARDING  SEDIMENTATION. 
In  the  production  of  the  rocks  next  taken  up,  moving  water 
plays  so  prominent  a  part  that  its  general  laws  are  described  by 
way  of  necessary  introduction.  All  streams  or  currents  charged 
with  suspended  materials  exercise  a  sorting  action  during  the 
deposition  of  their  loads.  With  materials  of  the  same  density  the 
sorting  will  grade  the  deposit  according  to  the  sizes  of  the  particles. 
With  materials  of  different  densities,  smaller  particles  of  heavier 
substances  will  be  mixed  with  larger  particles  of  lighter  ones. 
Assuming  a  swift  current,  we  readily  see  that,  when  it  slows  up, 
the  large  and  heavy  fragments  drop  first  of  all ;  then  the  smaller 
fragments  of  the  heavier  materials  and  the  larger  ones  of  those 
successively  lighter,  until  at  last  the  smallest  particles  of  the  light- 
est alone  remain  in  suspension.  It  is  also  important  to  bear  in 
mind  that  the  transporting  ability  of  a  current  varies  with  the  sixth 
power  of  the  velocity.  Thus  if  we  have  a  current  of  proper 
velocity  to  move  a  cube  of  quartz  one  inch  on  the  edge,  and  then 
double  the  velocity,  the  faster  current  can  move  one  sixty-four 
times  as  large ;  that  is  *bur  times  as  long,  or  four  inches  on  the 
edge.  An  appreciation  of  this  law  makes  the  size  of  boulders 
moved  by  many  streams,  in  times  of  flood,  less  surprising.  On  the 
other  hand,  when  the  suspended  material  becomes  excessively  fine, 
the  ratio  of  its  surface  to  its  volume  is  so  extremely  high  that 
adhesion,  or  chemical  action  akin  to  hydration,  or  some  other 
influence  not  well  understood,  operates  in  pure,  fresh  waters,  so  as 
to  practically  render  sedimentation  impossible,  even  if  the  medium 
be  perfectly  quiet.  W.  M.  Brewer  has  shown  by  a  series  of  ex- 
periments with  all  sorts  of  clays,  lasting  over  many  years,  that  if 
we  introduce  into  such  an  emulsion  a  mineral  acid  or  a  solution 


THE  AQUEOUS  AND  EOLIAN  ROCKS.  95 

of  salt  or  of  some  alkali,  the  turbidity  clears  with  remarkable 
quickness.  When,  therefore,  sediment-laden  streams  flow  into  the 
ocean,  or  into  salt  lakes,  even  the  finest  part  of  their  load  speedily 
settles  out. 

While  we  may  state  thus  simply  the  laws  of  sedimentation,  we 
must  not  expect  in  Nature  such  well-sorted  and  differentiated 
results  as  would  at  first  thought  appear  to  be  the  rule.  Of  rivers 
and  shore  currents  —  the  two  great  transporting  agents  —  the 
former  are  subject  to  floods  and  freshets,  giving  enormously  in- 
creased efficiency  for  limited  periods,  and  again  to  droughts,  with 
the  same  at  a  minimum.  Hence  varying  sediments  overlap  and 
are  involved  together.  Eddies  and  quiet  portions  in  the  streams 
themselves  contribute  further  confusion,  and  an  intermingling  of 
coarse  and  fine  materials.  Shore  currents  have  parallel  increases 
of  violence  in  times  of  high  wind  and  storms,  and  sink  again  in 
times  of  calm. 

Eolian  deposits  are  subject  to  even  greater  fluctuation,  and  their 
irregularities  are  more  pronounced  than  those  of  the  true  Aqueous. 
Both  these  classes  of  rocks  are  marked  by  a  more  or  less  perfect 
arrangement  of  their  materials  in  layers.  The  layers  give  rise  to 
regular  beds  in  deposits  from  quiet  and  uniform  currents,  and, 
although  in  those  from  swift  ones  they  are  very  irregular,  as 
explained  above,  nevertheless,  bedding,  or  stratification,  is  in  the 
highest  degree  characteristic  of  the  Aqueous  and  Eolian  grand 
division. 

When  in  the  presence  of  these  sedimentary  rocks  in  the  field, 
the  observer  should  always  appreciate  that  they  reproduce  past 
conditions,  and  that  they  indicate  the  former  presence  of  water, 
either  in  a  state  of  agitation  and  with  high  transporting  power  for 
the  coarse  varieties,  or  as  quiet  reaches  in  which  were  laid  down 
the  finer  deposits.  Rightly  approaching  and  interpreting  them, 
we  may  see  that  the  ocean  has  advanced  across  the  land  in  times 
of  submergence,  leaving  behind  its  widening  trail  of  shore  gravels, 
now  conglomerates  ;  that  these  have  been  followed  up  and  buried 
first  by  fine  offshore  sediments,  and  later  by  the  remains  of  organ- 
isms now  appearing  as  limestones,  until  succeeding  elevation  causes 
the  waters  again  to  retreat  and  prepare  the  way  for  another  "  cycle 
of  deposition." 


96  A   HAND  BOOK  OF  ROCKS. 

GRAVELS  AND  CONGLOMERATES. 

Loose  aggregates  of  rounded  and  water-worn  pebbles  and  bould- 
ers are  called  gravels,  and  when  they  become  cemented  together 
into  coherent  rocks  they  form  conglomerates.  Sand  almost  always 
fills  the  interstices.  Silica,  calcite  and  limonite  are  the  commonest 
cements.  The  component  pebbles  are  of  all  sorts  of  rock  depend- 
ing on  the  ledges  that  have  supplied  them,  hard  rocks  of  course 
predominating.  Rounded  fragments  of  vein  quartz  are  especially 
frequent.  Gravels  and  conglomerates,  if  of  limited  extent,  indicate 
the  former  presence  of  swift  streams  ;  if  of  wide  area  they  suggest 
the  former  existence  of  sea  beaches  and  the  advance  of  the  sea 
over  the  land.  Component  pebbles  are  of  course  older  than  the 
conglomerate  itself,  and  if  igneous,  they  may  establish  the  age  of 
the  intrusion  as  older  than  the  conglomerate.  Fossiliferous  boul- 
ders prove  the  age  of  the  conglomerate  as  later  than  their  parent 
strata.  Under  favorable  circumstances  gravels  may  be  cemented 
to  conglomerates  in  a  comparatively  few  years.  Conglomerates 
are  exclusively  aqueous.  Gravels  and  conglomerates  graduate  by 
imperceptible  stages  into  pebbly  sands  and  sandstones,  and  these 
into  typical  sands  and  sandstones.  Notably  unsorted  aggregates 
of  relatively  large  and  more  or  less  angular  boulders  in  fine  sands 
or  clay  indicate  glacial  action. 

Metamorphism. — Under  dynamic  stresses,  especially  in  the 
nature  of  pressure  and  shearing,  the  pebbles  of  a  conglomerate 
may  be  flattened  and  rolled  out  into  lenses,  and  these  are  often 
observed.  If  the  pebbles  are  feldspathic  as  is  the  case  in  those 
from  granite  ledges,  and  if  the  interstitial  filling  is  aluminous  and 
not  purely  quartzose  as  in  the  commonest  cases,  conglomerates, 
when  recrystallized,  may  pass  into  augen-gneisses  with  their  char- 
acteristic "augen,"  or  "  eyes  "  of  feldspar  and  quartz  that  but  faintly 
suggest  their  original  character.  Excessive  metamorphism  may 
further  develop  types  closely  simulating  granite,  forming  thus  the 
so-called  "  recomposed  granite  "  of  the  Lake  Superior  regions. 

Occurrence.  —  Gravels  are  too  familiar  to  require  further  refer- 
ence. Conglomerates  are  met  in  all  extended  sedimentary  series. 
Our  greatest  one  lies  at  the  base  of  the  productive  Coal  Measures 
of  Pennsylvania  and  adjacent  States.  It  is  properly  called  the 


SiO, 
99.78 
98.84 
99.62 
99-47 
95.8S 
94-73 
91.67 
82.52 
69.94 

S; 

FeO 
AI2O,       Fe,O, 
O.22 
0.17        0.34 
...          0.13 
O.I7        O.I2 
2.64 
0.36        2.64 
6.  92        tr. 
7-07      3-55 
I3-I5      2-48 

iNDS  AND  SANDSTONES. 

MnO       CaO         MgO        K,O       Na,O 

Loss. 

tr. 

tr. 

tr. 

0.23 

0.7 

1.42 
0.70 

0.90 

o.Si 
0.38 
0.28 
1-83 
3-09 

0.50 
0.08 
0.36 
o-34 
tr. 
tr. 

0.07      0.15 

tr.          tr. 
3.30      5.43 

0.12 

0-45 

0.83 
1.17 
3.61 

I.OI 

THE  AQUEOUS  AND   EOLIAN  ROCKS.  97 

"  Great  Conglomerate."  Remarkable  ones  with  squeezed  pebbles 
are  met  in  the  Marquette  iron  region  of  Michigan.  In  Central 
Massachusetts  there  is  an  augen-gneiss  that  has  been  derived  from 
a  Cambrian  conglomerate.  It  is  quarried  at  Munson,  and  sold  as 
granite,  and  is  a  widely  known  building  stone.  Around  Narragan- 
sett  Bay,  R.  I.,  are  conglomerates,  in  part  at  least  of  Carboniferous 
age,  in  all  stages  in  the  progress  to  gneiss. 


Sp.  Gr. 

i. 

2. 

3- 

4.  99.47        0.17        0.12  ...          0.90        0.50        0.07        0.15        0.12  2.647 

5-        95-85  2-64  ...       0.81      0.08        0.45       (2.245) 

6. 

7.        91.67      6.92        tr.          ...       0.28      0.34         1.17        2.240 

(2.36) 

I.  Sand  from  Cambrian  Quartzite,  Chesire,  Mass.,  S.  Dana  Hayes,  Mineral  Re~ 
sources,  1 883-' 84,  p.  962.  2.  Oriskany  Sandstone,  Juniata  Valley,  Penna.  A.  S. 
McCreath,  Idem.  3.  Siluro-Cambrian  saccharoidal  sandstone,  Crystal  City,  Mo.  An- 
alyzed by  Glass  Co.,  0.22  not  determined,  Idem.  4.  Novaculite,  Rockport,  Ark.  R. 
N.  Brackett,  for  L.  S.  Griswold,  Geol.  Ark.,  1890,  III.,  161.  5.  Salmon-red  Triassic 
Sandstone,  Glencoe,  Colo.  Quoted  by  G.  P.  Merrill,  "Stones  for  Building  and  Deco- 
ration," p.  420.  The  Sp.  Gr.  is  of  one  from  Ralston,  near  by.  6.  Cambrian  red 
Sandstone,  Portage  Lake,  Mich.,  Idem.  7  Light-gray  sub-carboniferous  sandstone, 
near  Cleveland,  Ohio,  Idem.  8.  Olive-green  carboniferous  sandstone,  Dorchester,  N. 
B.,  Idem.  9.  Red  triassic  sandstone  (brownstone,  arkose),  Portland,  Conn.,  Idem. 

Comments  on  the  Analyses.  —  The  first  three  illustrate  the  purity 
of  the  sand  in  exceptional  cases.  We  may  properly  infer  that  the 
sediments  were  derived  either  from  preexisting  sandstones,  that 
had  already  been  once  sorted  and  separated  from  their  aluminous 
ingredients,  or  from  excessively  weathered  and  kaolinized  quartzose 
rocks,  such  that  the  feldspar  had  entirely  passed  into  clay,  and  had 
been  eliminated  in  deposition.  No.  4  is  a  novaculite,  and  is  an  ex- 
cessively fine,  fragmental  deposit.  Nos.  5,  6  and  9  are  red  sand- 
stones, and  indicate  the  comparatively  small  percentage  of  iron 
oxides  that  may  cause  a  deep  coloration.  No.  7  is  free  from  iron, 
but  has  some  aluminous  material,  evidently  a  very  pure  clay,  from 
the  lack  of  iron.  No.  8  has  its  iron  as  protoxide,  for  the  rock  is  ? 
green  variety.  Its  manganese  oxide  is  worthy  of  remark.  No. 
7 


98  A   HAND  BOOK  OF  ROCKS. 

9  is  a  feldspathic  sandstone,  or  arkose,  whose  analysis,  except  that 
the  A12O3  is  low  and  the  CaO  rather  high,  might  answer  for  a 
granite. 

The  specific  gravity  of  sandstones  varies  widely.  Quartz  itself 
is  2.6-2.66,  and  specially  dense  sandstones  reach  2.5,  but,  being 
characteristically  porous  rocks,  the  usual  range  is  2.2—2.4.  They 
often  go  lower  and  many  even  reach  1.8. 

Mineralogical  Composition,  Varieties.  —  The  mechanical  sedi- 
ments whose  predominant  particles  are  finer  than  pebbles,  and  yet, 
in  most  cases,  of  notable  size,  are  grouped  under  this  head.  They 
are  found  in  all  stages  of  coherence,  from  loose  sands  to  exces- 
sively hard,  metamorphic  rocks  called  quartzites.  Quartz  is  much 
the  commonest  mineral  that  contributes  the  grains,  as  it  is  the  most 
resistant  of  the  common  rock-making  minerals.  In  river  sands  the 
grains  are  angular,  but  in  those  -continually  washed  together  on  a 
sea  beach,  they  become  more  or  less  rounded.  Garnets,  magne- 
tite, zircon  and  other  hard  and  resistant  minerals  are  widely  dis- 
tributed in  small  quantities.  Feldspathic  sands  also  occur,  and 
when  they  are  compacted  into  firm  rock  they  are  called  arkose. 
As  in  the  conglomerates,  the  cementing  materials  of  sandstones  are 
silica,  calcite  and  limonite,  but  in  many  the  character  or  cause  of 
the  bond  is  rather  obscure.  Those  with  siliceous  cement  yield  the 
most  durable  stone  for  structural  purposes  ;  those  with  ferruginous 
afford  the  greatest  range  of  colors,  such  as  olive-green,  yellow, 
brown,  and  red.  Calcareous  cements  may  be  detected  by  their 
feeble  effervescence.  Sandstones  entirely  formed  of  calcareous  frag- 
ments are  known,  but  are  described  under  limestone. 

A  curious  and  exceptional  rock  is  the  novaculite,  that  is  exten- 
sively developed  in  Arkansas.  It  was  long  thought  to  be  allied  to 
the  cherts,  which  it  much  resembles,  but  microscopic  investigation 
has  led  Griswold  to  determine  it  to  be  a  finely  fragmental  deposit  of 
quartz  grains,  practically  a  siliceous  ooze.  In  fineness  it  is  parallel 
with  the  clays,  but  it  contains  little  else  than  silica. 

Aqueous  sandstones  generally  exhibit  well-marked  bedding 
planes,  although  cases  are  familiar  in  which  the  bedding  is  exces- 
sively coarse  and  the  layers  are  extremely  thick.  Swirling  eddies 
in  the  original  stream  or  currents  give  rise  to  cross-bedding  and 
various  irregularities.  In  fact,  all  the  phenomena  of  beaches  and 


THE  AQUEOUS  AND  EOLIAN  ROCKS.  99 

stream -bottoms,  such  as  ripple-marks,  worm-borings,  shells,  etc.; 
are  preserved  in  sandstones. 

Eolian  sands  are  usually  of  aqueous  deposition  in  their  original 
condition,  but  they  are  afterwards  taken  up  by  the  wind  and  driven 
along  as  dunes  and  dust  into  more  or  less  remote  districts.  When 
they  finally  reach  a  state  of  rest  and  consolidate,  they  have  very 
irregular  stratification,  cross-bedding,  swirling  curves,  pinching  and 
swelling  layers  and  other  characteristic  phenomena.  Finer  varieties 
afford  a  surface  deposit  that  is  generally  called  "  loess,"  and  that 
may  lack  all  stratification.  More  or  less  water-transported  mate- 
rial is  also  intermingled,  making  the  term  one  of  not  particularly 
sharp  definition.  This  mixed  character  has  made  the  loess  of 
many  localities  a  rather  puzzling  geological  problem.  It  is  alwayf 
loosely  textured  and  is  important  in  its  relations  to  agriculture. 

Sands  and  sandstones  pass  by  insensible  gradations  into  the 
varieties  in  the  upper  line  of  the  series  shown  in  the  tabulation  on 
p.  92  by  the  increasing  admixture  of  clayey  or  argillaceous  ma- 
terials. The  base  is  kaolin,  A12O3,  2SiO2,  2H2O,  a  mineral  that 
forms  microscopic,  scaly  crystals  and  that  has  the  property  of  plas- 
ticity, and  this  property  it  lends  to  the  last  members  of  the  series, 
which  in  exceptional  cases  may  contain  little  else.  The  lower  series 
passes  gradually  into  the  fragmental  limestones,  by  the  increasing 
admixture  of  calcite. 

Metamorphism. — The  purer  sandstones  in  metamorphism  yield 
quartzites  which  are  denser  and  harder  than  their  originals,  because 
by  deposition  of  cementing  quartz,  the  fragmental  grains  are  very 
firmly  bound  together.  The  later  deposited  quartz  often  con- 
forms to  the  optical  and  crystallographic  properties  of  the  grain 
around  which  it  crystallizes.  No  sharp  line  divides  sandstones  and 
quartzites ;  they  shade  imperceptibly  into  one  another.  Less  pure 
sandstones,  if  crushed  and  sheared  in  the  metamorphic  process, 
yield  siliceous  or  quartz  schists  from  the  development  of  micaceous 
scales  between  the  grains.  Flexible  sandstone  or  itacolumite  has 
been  thought  to  owe  to  them  its  property  of  bending,  but  it  is  now 
generally  attributed  to  the  interlocking  of  grains. 

Occurrence.  —  Sandstones  are  so  common  in  all  extended  geolog- 
ical sections  as  to  deserve  slight  special  mention.  Next  to  lime- 
stones they  are  the  most  widely  used  of  building  stones  in  quantity, 


IOO 


A  HAND  BOOK  OF  ROCKS. 


although  the  money  value  of  the  annual  output  of  granite  is  greater. 
The  Potsdam  sandstone  of  the  Cambrian  in  New  York  and  on  the 
south  shore  of  Lake  Superior  is  extensively  quarried.  Other 
prominent  sandstones  are  the  Medina  of  New  York,  the  Berea  grit 
of  the  Subcarboniferous  of  Ohio  ;  and  the  red  and  brown  Triassic 
sandstones  both  of  the  Atlantic  seaboard  and  the  Rocky  Mountains. 

ARGILLACEOUS  SANDSTONE,  SHALE,  CLAY. 


(a)  SiO,      (b)  SiO,         AlaO3 

FeO 
Fea03 

CaO 

MgO    KaO,  NaaO    (a)HaO(b)HaO 

Shale. 

I. 

69.92 

23.46 

O.2O 

0.48 

0.40 

1-43 

3-84 

2. 

67.29 

15.85 

6.16 

0-95 

0.19 

8.71 

3- 

64.37 

19-73 

9.07 

0.82 

2.32 

3-78 

4- 

62.86 

20.65 

9-21 

0.48 

0-34 

6.26 

5- 

58.45 

21.96 

8-43 

1.05 

i-57 

4.00 

6.51 

6. 

43.13 

40.87 

3-44 

8.90 

5-32 

2.42 

O.2O 

Brick  Clay. 

7- 

8I.7I 

9.81 

3-80 

0.48 

0.26 

3-91 

8. 

75-88 

11.22 

5-04 

0.48 

0-35 

6.76 

9- 

65-14 

13.38 

7.65 

2.18 

2.36 

8.51 

10. 

62.00 

I8.IO 

9.11 

5-66 

n. 

57.80 

22.6O 

1.85 

1.07 

12.68 

12. 

53.77 

20.49 

9-23 

2.04 

4.22 

9.60 

13- 

45-73 

29.69 

6.86 

0.44 

I.OI 

3-42 

12.86 

Potter's  Clay. 

14.     27.68 

36.58 

22.95 

1.28 

0.45 

0-37 

1.96 

6-74 

2.05 

15.     42.28 

18.02 

24.12 

1.46 

0.59 

0.68 

2.42 

7-77 

0.86 

Fire  Clay. 

16. 

61.60 

28.38 

0.52 

0.46 

0.36 

5-08 

17.     38.10 

12.70 

31-53 

0.92 

tr. 

0.40 

11.30 

2.50 

1  8. 

45-29 

40.07 

1.07 

0.26 

0.08 

0.48 

13-18 

Residual  Cl 

ay. 

19. 

55-42 

22.17 

8.30 

0.15 

1.45 

2.49 

9.86 

20. 

33-55 

30.18 

1.98 

3-89 

0.26 

i-57 

10.72 

Kaolinite. 

21.  46.50  39-57       .«  »3-93 

NOTE.  Where  two  values  of  SiO,  are  given,  the  first  is  the  combined  silica,  i.  e. 
chiefly  in  kaolinite,  and  the  second  the  free  silica,  which  is  practically  comminuted 
quartz.  Under  H,O,  where  two  values  are  given,  the  first  is  combined  water,  likewise 
chiefly  in  kaolinite,  the  second  is  the  free  water,  which  has  simply  been  absorbed. 

I.  Haydensville,  Hocking  Co.,  O.  Quoted  by  H.  Ries,  XVI.  Ann.  Rep.  Director 
U.  S.  Geol.  Survey,  Part  IV.,  p.  572.  2.  Hornellsville,  Steuben  Co.,  N.  Y.,  Ibid., 
572.  3.  Kansas  City,  Mo.,  Ibid.,  570.  4.  Red  Shale,  Sharon,  Mercer  Co.,  Pa., 
Ibid.,  572.  5.  Leavenworth,  Kan.,  Ibid.,  570.  6.  Clinton,  Vermilion  Co.,  Ind., 
Ibid.,  570.  7.  Washington,  Davies  Co.,  Ind.,  Idem,  566.  8.  Salem,  Washington 
Co.,  Ind.,  Idem,  566.  9.  Red  Clay,  Plattsburg,  Clinton  Co.,  N.  Y.,  Ibid.,  568. 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         101 

10.  Red  Clay,  Lasalle,  111.,  Ibid.,  564.  II.  Rondout,  N.  Y.,  Ibid.,  568.  12.  Brown 
Clay,  Fisher's  Is.,  N.  Y.,  Ibid.,  568.  13.  Hooversville,  Somerset  Co.,  Pa.,  Ibid., 
568.  14.  Akron,  O.,  Ibid.,  562.  15.  East  Liverpool,  O.,  Ibid.,  562,  alsoTiO,,  1.20. 
16.  Woodbridge,  N.  J.,  Ibid.,  556.  17.  Cheltenham,  Mo.,  Ibid.,  556.  18.  Wood- 
land, Pa.,  Ibid.,  556.  19.  Morrisville,  Calhoun  Co.,  Ala.,  Ibid.,  574.  20.  Near 
Batesville,  Ark.,  Ibid.,  574,  also  PjOj,  2.53.  21.  Pure  Kaolinite— A1,O,  2SiO,,  2H,O. 

Comments  on  the  Analyses.  —  The  analyses  are  significant  when 
compared  with  those  of  the  sandstones  on  p.  97.  It  appears  at 
once  that  there  is  a  great  decrease  in  silica,  and  a  great  increase  in 
alumina,  and,  as  a  rule,  in  all  the  other  bases  and  water.  Among 
themselves  there  is  wide  variation,  but  by  using  No.  21,  as  indi- 
cating pure  kaolin,  it  is  possible  to  infer  how  much  quartz  sand  is 
mingled  with  the  clay,  due  allowance  being  made  for  the  fragments 
of  unaltered  feldspar,  as  shown  by  the  alkalies,  for  silicates,  hydrous 
and  anhydrous,  involving  iron,  lime  and  magnesia,  and  for  carbo- 
nates of  lime,  magnesia  and  iron.  Shales  and  brick  clays  are  shown 
to  be  comparatively  impure  admixtures  of  kaolinite  and  quartz ; 
potter's  clay  is  much  less  so,  and  fire-clay  is  little  else  than  these 
two.  No.  1 8  is  practically  pure  kaolin. 

Mineralogical  Composition,  Varieties. — The  argillaceous  sand- 
stones have  a  finer  grain  than  the  sandstones  proper,  and  tend  to 
form  thin  but  tough  beds.  They  find  their  best  examples  in  the 
flagstones  of  our  eastern  cities.  Shales  lack  this  coherence  and 
break  readily  into  irregular  slabs  and  wedge-shaped  fragments  of 
no  notable  size.  As  sands  give  rise  to  sandstones,  so  on  harden- 
ing and  drying,  muds  and  silts  yield  shales.  Shales  show  all  grades 
from  gritty  and  coarse  varieties  to  fine  and  even  ones  approximat- 
ing clays.  The  finer  shales  when  ground  have  the  same  plasticity 
as  clay,  and  are  often  moulded  and  baked  into  brick,  especially  of 
the  vitrified  kinds  for  paving.  Shales  may  be  black  from  bitumi- 
nous matter  in  them,  and  are  then  described  as  "  bituminous." 
They  grade  into  cannel  coals,  but  great  areas  of  them  such  as  the 
Genesee  Shale  of  New  York  and  the  Huron  Shale  of  Ohio,  have 
as  much  as  8  to  20  %  hydrocarbons  and  yield  quite  copious  prod- 
ucts on  distillation. 

As  the  particles  of  quartz  become  finer  and  finer  and  not  too 
abundant,  the  plasticity  of  the  kaolinite  presently  asserts  itself  so  that 
the  shales  pass  into  clays.  In  the  most  even  and  homogeneous 
grades,  they  show  but  slight  grit  to  the  teeth,  but  in  coarser 


102  A   HAND  BOOK  OF  ROCKS. 

varieties  they  are  decidedly  gritty  even  to  the  fingers.  They  are 
often  separated  into  thin  beds  by  layers  of  sand  that  mark  the 
times  of  freshets  during  their  formation  and  the  attendant  deposi- 
tion of  coarse  material.  Clays  of  earlier  geological  date  are  hard 
and  dense  rocks  and  must  be  ground  before  use.  Such  are  the 
fire-clays  immediately  beneath  Carboniferous  coal-seams.  Clays 
are  blue,  red  and  brown  according  to  the  state  of  the  iron  oxide, 
whether  ferrous  or  ferric,  or  they  may  be  nearly  white  when  it 
fails.  The  less  pure  brick  clays  as  shown  by  the  analyses  contain 
oxides  of  iron,  calcium,  magnesium  and  of  the  alkalies  in  quantity, 
but  fire-clays  practically  lack  these. 

As  contrasted  with  the  transported  or  sedimentary  clays  just 
mentioned,  there  are  residual  clays  that  result  from  the  decay  of 
impure  limestones  and  that  are  found  on  weathered  outcrops. 
They  are  very  impure  and  variable  in  composition,  but  they  are 
markedly  plastic. 

Metamorphism.  —  In  metamorphic  processes  shales  become  com- 
pacted and  oftentimes  silicified.  Their  lack  of  homogeneity  causes 
them  to  yield  irregularly  breaking  and  very  tough  rocks  called 
graywackes,  which  differ  only  in  greater  hardness  from  their 
unaltered  originals.  Excessively  silicified  shales  are  called  phtha- 
nites  and  are  important  in  the  Coast  Range  of  California.  Shales 
also  under  shearing  stresses  and  attendant  mineralogical  reorgani- 
zation pass  into  schists  of  various  kinds,  such  as  quartz-schist, 
mica-schist  and  possibly  hornblende-schist.  G.  F.  Becker  even 
mentions  rocks  derived  from  them  that  are  mineralogical  ly  like 
diabases  and  diorites,  but  their  recognition  is  a  matter  for  micro- 
scopic study.  Clays  under  shearing  stresses  develop  new  cleavages 
without  regard  to  their  original  bedding  and  from  the  homogeneous 
character  of  the  original  and  the  perfection  of  the  cleavage,  slates 
result,  which  are  of  great  practical  importance. 

Occurrence.  —  Shales  and  clays  are  such  common  members  of 
extended  geological  sections  as  to  deserve  no  special  mention. 
They  are  often  a  thousand  feet  or  more  in  thickness  and  cover 
great  areas. 


THE  AQUEOUS  AND  EOLIAN  ROCKS.          103 
CALCAREOUS  SANDSTONES,  MARLS. 

HsOor 
,0          CO,         Loss. 

3.26 


C»lc. 
Sandst 

.    SiO, 

A1,O, 

FeO 
Fe,O3 

CaO 

MgO 

K20       Na,0 

I. 

79.19 

3-75 

7.76 

3-20 

2. 

38.41 

5-77 

1.79 

20.08 

8.82 

0.12 

0.29 

Calc. 

Shales. 

3- 

39-70 

26.83 

19.28 

2.43 

5.II 

4- 

28-35 

12.37 

21.47 

8.24 

5-73 

Marls. 

5- 

43-70 

25.00 

8.85 

2-33 

... 

... 

6. 

38.70 

IO.2O 

18.63 

9.07 

1.50 

3-65 

... 

7- 

28.78 

".63 

2.96 

24.50 

2.91 

2.12 

... 

5.40     9.21 

6.14      lo.oo 
22.66        4.18 

I.  Calcareous  sandstone,  Flagstaff,  Ariz.  Quoted  by  G.  P.  Merrill,  Stones  for 
Building  and  Decoration,  420.  2.  Calcareous  sandstone,  Jordan,  Minn.,  Idem.  3. 
Genesee  Shale,  Mt.  Morris,  N.  Y.,  supplied  by  H.  Ries.  4.  Niagara  Shale,  Rochester, 
N.  Y.,  H.  T.  Vulte,  analyst.  Supplied  by  H.  Ries.  5.  Cretaceous  Marl,  Hop  Brook, 
N.  J.,  Geol.  of  N.  J.,  1868,  419 ;  also  P,O5,  2.18.  6.  Cretaceous  Marl,  Red  Bank, 
N.  J.,  Idem,  418;  also  P,O5,  1.14,  SO,  0.14.  7.  Subcarboniferous  marl,  Bowling 
Green,  Ky.,  Ky.  Geol.  Surv.,  Chem.  Analyses  A,  Part  3,  90;  also,  P2O5,  0.25. 

Comments  on  the  Analyses.  —  The  analyses  illustrate  in  a  very 
suggestive  way  the  passage  of  these  mechanical  sediments  into  im- 
pure limestones.  The  gradual  intermingling  of  more  and  more  of 
shells  and  other  remains  of  organisms  brings  it  about.  The  high 
P2O5  of  the  marls,  as  cited  under  the  references,  is  worthy  of  re- 
mark. It  is  to  be  appreciated  that  the  lime  and  magnesia  and 
some  of  the  iron  of  the  analyses  are  to  be  combined  with  CO2,  even 
though  the  CO2  is  not  mentioned. 

Mineralogical  Composition,  Varieties.  —  Calcareous  sandstones 
are  practically  sandstones  with  rich  calcareous  cement,  or  with  a 
large  amount  of  organic  fragments  intermingled  with  the  prevailing 
quartz  sand.  They  are  passage  forms  to  the  fragmental  limestones. 
Calcareous  shales  derive  their  lime  partly  from  the  fine  organic 
sediment  that  is  deposited  with  the  siliceous  and  aluminous  particles 
and  partly  from  contained  fossils.  Beds  of  these  rocks  are  partic- 
ularly favorable  layers  for  the  discovery  of  the  latter,  and  often 
break  the  monotonous  barrenness  of  a  geological  section  composed 
of  ordinary  shales.  Marls,  strictly  speaking,  are  calcareous  clays, 
and  originate  in  typical  cases  by  the  deposit  of  limy  slimes  along 
with  the  aluminous.  The  lime  destroys  the  plasticity  of  the  clay 
and  yields  a  crumbling  rock,  often  richly  provided  with  fossils  and 
of  value  as  a  fertilizer.  Grains  of  glauconite,  the  green  silicate  of 
potash  and  iron,  are  at  times  present,  and  characterize  the  so-called 


104  'A  HAND  BOOK  OF  ROCKS. 

"  green  sands  "  which  are  valuable  as  fertilizers.  The  term  marl  is 
somewhat  loosely  used  in  its  applications,  and  moderately  coarse 
calcareous  sands,  and  even  beds  which  show  but  small  percentages 
of  lime  on  analysis  are  designated  by  it  in  the  States  along  the 
Atlantic  seaboard  from  New  York  south.  It  is  clear  that  marls 
are  intermediate  rocks  between  clays  and  impure  earthy  limestones. 

MetamorpJusm.  —  The  rocks  of  this  group  are  altered  in  meta- 
morphic  processes  to  schistose  forms,  not  so  essentially  different 
from  those  resulting  from  the  common  aluminous  shales  and  clays, 
except  that  the  richness  in  lime  facilitates  the  production  of  minerals 
requiring  it.  The  marls,  when  high  in  lime,  behave  like  impure 
limestones,  and  are  prolific  sources  of  silicates.  Marls  are,  how- 
ever, much  more  common  in  later  and  unmetamorphosed  formations 
than  in  older  ones,  although  it  may  be  that  in  the  latter  they  have 
yielded  some  schistose  derivatives  not  readily  traceable  back  to 
them. 

Occurrence.  —  Calcareous  sandstones  and  shales  are  met  as  occa- 
sional beds  in  series  of  the  more  abundant,  distinctively  aluminous 
varieties.  Marls  are  chiefly  developed  in  the  Cretaceous  and  Tertiary 
strata  of  the  Atlantic  seaboard  and  around  the  Gulf  of  Mexico. 
Freshwater  ones  are  not  lacking  in  the  Tertiary  lake  basins  of  the 
West. 


CHAPTER  VIII. 

LIMESTONES;  ORGANIC  REMAINS  NOT  LIMESTONES;  ROCKS 

PRECIPITATED  FROM  SOLUTION.     DETERMINATION  OF 

THE  AQUEOUS  AND  EOLIAN  ROCKS. 


II.    LIMESTONES. 


SiO,      A1,O, 

Fe,O,     FeO 

CaO 

MgO 

Living  Organisms. 

I.   (Coral) 

54-57 

2.   (Reef-rock) 

53-82 

I.OI 

3.   (Lagoon  Sed.  ) 

54-58 

0.85 

4.   (Coral) 

44.96 

3.87 

5.   (Oyster  Shells) 

44.4 

1-3 

Calcite. 

6.  Pure  Mineral. 

56. 

Dolomite. 

7.  Pure  Mineral. 

30.43 

21.72 

Marine  Limestones. 

8.        0.63 

0.55 

55-6 

0.23 

9- 

1.06 

53-78 

0-34 

10. 

1.25 

53-89 

O.  IO 

ii.        1.84      0.64 

1.82 

51-40 

2.23 

12.        12.34 

7.00 

44.41 

0.44 

13.          3.77        0.08 

6.80 

33-79 

15.32 

14. 

0-55 

29.54 

21.  08 

Waterlime. 

15.      18.34 

7-49 

37.60 

1.48 

16.      15.37 

11.38 

25.70 

12.44 

CO,     H,0     Insol.     CaCO,    MgCO, 


2-54 


35-4     14-5 


97.46 

96.11  2.13 

97-47  t-79 

So.  29  8.14 

(79-28)  (2.73) 


54-35    45.65 


41.19 

0.90      I.I3 
0.27 

99-30 
96.04 
96.24 
91.80 

0-49 
0.72 
O.2I 

4.68 

42.21 

1.82      0.60 

79-30 
60.35 
52.75 

0.92 
32.61 
44-28 

3-94 
1.20 

67.14 
45-91 

2.90 
26.14 

1.24  43.72   31.60   22.24 


1.49 
4-64  31.28 


96.71 
61.07 


0.31 
0-23 


Siliceous. 

17.  1. 20  17.69     10.59 
Freshwater  Limestone. 

18.  0.37    54.16      0.15    43.68 

19.  1.83  O.22      34.2O        O.  II      26.79 

Travertine. 

20.  0.08  0.15  53.83       0.90    41.79     1.43  94.97      0.43 

I.  Stag's  horn  coral  (Millepora  alcicornis),  S.  P.  Sharpless,  Amer.  Jour.  Sci., 
Feb.,  1871,  168.  2.  Bermuda  coral  reef  rock.  A.  G.  HSgbom,  Neuesjahrb.,  1894, 
I.,  269.  3.  Bermuda  coarse  lagoon  sediment,  Idem.  4.  Average  of  14  analyses  of  the 
coral  Lithothamnium  from  localities  the  world  over,  Idem,  272.  5.  Oyster  shells,  Geol. 
of  New  Jersey,  1868,  405.  6.  Calculated  from  CaCO,.  7.  Calculated  from  CaCOs 
MgCOj.  8.  Crystalline  Siluro-Camb.  limestone,  Adams,  Mass.,  E.  E.  Olcott  for  Marble 
Co.  9.  Limestone,  Bedford  limestone,  Ind.  Quoted  by  T.  C.  Hopkins,  Mineral  In- 
dustry, 1894,  505.  10.  Solenhofen  lithographic  stone.  Quoted  by  G.  P.  Merrill, 

105 


106  A  HAND  BOOK  OF  ROCKS. 

Stones  for  Building  and  Decoration,  415.  1 1.  Limestone,  Hudson,  N.  Y.,  Th.  Egleston. 
12.  Trenton  limestone,  Point  Pleasant,  Ohio,  vide  No.  IO.  13.  Surface  Rock,  Bonne 
Terre,  Mo.,  J.  T.  Monell,  unpublished.  14.  Limestone,  Chicago,  T.  C.  Hopkins, 
Mineral  Industry,  1895,  508.  15.  Hydraulic  limestone,  Coplay,  Penn.  Quoted  by 
W.  A.  Smith,  Mineral  Industry,  1893,  49.  1 6.  Hydraulic  limestone,  Rosendale,  N. 
Y.,  Idem.  17.  Siliceous  limestone,  Chicago,  111.,  vide  No.  14.  18.  Miocene  lime- 
stone, Chalk  Bluffs,  Wyo.,  R.  W.  Woodward,  4Oth  Parallel  Surv.,  I.,  542.  19.  Eocene 
limestone,  Henry's  Forks,  Wyo.,  B.  E.  Brewster,  Idem.  20.  Travertine,  below  Hotel 
Terrace,  Yellowstone  Park,  J.  E.  Whitfield,  for  W.  H.  Weed,  gth  Ann.  Rep.  Dir. 
U.  S.  Geol.  Surv.,  646. 

Comments  on  the  Analyses.  — The  first  three  analyses  and  the  fifth 
indicate  that  the  calcareous  parts  of  living  organisms  are  quite  pure 
calcium  carbonate.  The  fourth  analysis  is  of  that  species  of  coral 
which,  so  far  as  we  know,  is  highest  in  magnesia.  Small  amounts 
of  calcium  phosphate  are  often  present  as  well,  some  shells  being 
richer  than  others.  Nos.  6  and  7  are  introduced  so  as  to  give  a 
basis  for  estimating  the  purity  of  the  following  limestones  :  Nos. 
8,  9  and  10  are  extremely  pure  varieties,  and  from  these,  as  a 
starting  point,  the  other  components  increase  in  one  analysis  and 
another.  No.  14  is  a  nearly  typical  dolomite.  Nos.  12  and  17  are 
highly  siliceous,  and  Nos.  1 5  and  16  are  both  strongly  argillaceous. 
The  last  two  are  closely  parallel  in  composition  with  marine  varie- 
ties. An  analysis  of  a  travertine  is  given  in  No.  20. 

It  at  once  appears  that  Nos.  13,  14,  16  and  17  are  far  higher  in 
magnesia  than  any  known  living  organism,  and  it  is  evident  that 
an  original  organic  deposit  must  have  undergone  an  enrichment  in 
magnesium  carbonate  to  bring  them  about.  Dana  suggested  many 
years  ago  that  coral  or  other  organic  sand,  when  agitated  in  sea- 
water,  probably  exchanges  a  part  of  its  calcium  for  magnesium, 
and  there  is  much  reason  to  think  that  it  does.  Otherwise,  the 
change  must  have  been  brought  about  by  magnesian  solutions  per- 
colating through  the  rock  and  altering  it  by  the  replacement  proc- 
ess called  dolomitization,  or  dolomization.  Much  of  the  silica,  no 
doubt,  results  from  radiolarians  and  sponge  spicules,  but  much 
also,  together  with  the  alumina,  from  fine  fragmental  sediments. 

Origin.  —  Much  the  greater  number  of  the  important  limestones 
are  of  marine  origin,  but  in  certain  geological  formations  fresh- 
water ones  are  well  developed.  The  calcareous  remains  of  organ- 
isms have  been  their  principal  source,  and  of  these  the  forami- 
nifera,  the  corals,  and  the  molluscs  have  been  the  chief  contributors. 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         107 

Their  shells  have  often  become  thoroughly  comminuted  to  a  calcare- 
ous slime  before  final  deposition,  so  that  the  resulting  rock  affords  no 
trace  of  organic  structure.  The  solubility  of  the  carbonate  of  lime 
aids  in  the  cementation  of  the  slime  to  rock  and  tends  to  efface  the 
organic  characters.  Limestones  pass  by  insensible  -gradations 
through  more  and  more  impure  varieties  into  calcareous  shales  and 
marls,  but,  as  a  rule,  they  are  deposited  in  deeper  water  than  the 
true  shales  and  sandstones.  This  conception  must  not  be  applied 
too  strictly,  because,  beyond  question,  a  depth  of  a  few  feet  has 
often  sufficed,  and  too  much  emphasis  has  often  been  placed  upon 
the  depth  regarded  as  necessary  for  limestones.  Coral  sands  ac- 
cumulate on  or  near  the  immediate  shore,  and  may  even  be  heaped 
up  by  the  wind. 

The  general  geological  relations  involved  in  the  deposition  of 
limestones  are  well  illustrated  in  the  accompanying  Fig.  41.     The 


FIG.  41.  Cross-section  of  a  fossil-coral  reef  at  Alpena,  Mich.,  showing  the  reef- 
coral  in  the  fan-shaped  pattern  ;  the  coarse  coral-sand  in  the  shaded  part ;  and  the  fine 
sand  shading  into  slimes  farther  away.  After  A.  W.  Grabau,  Annual  Kept.  Mich. 
State  Geologist,  1901,  176. 

reef  of  coral  grows  constantly  and  from  the  action  of  the  breaking 
waves  is  partially  comminuted  to  sand,  which  settles  on  the  flanks 
and  furnishes  a  place  of  residence  for  various  mollusca  whose  hard 
parts  contribute  also  to  the  growing  limestone.  The  finer  material  is 
transported  to  a  greater  distance  and  gradually  settles  out  as  slimes 
which  afford  dense  and  often  thin-bedded  varieties.  The  conditions 
for  the  deposition  of  the  latter  have  often  been  unfavorable  for 
organic  life,  and  it  frequently  happens  that  the  resulting  limestones 
are  devoid  of  fossils  except  in  the  vicinity  of  the  old  reef.  Based 
upon  the  varieties  just  outlined,  A.  W.  Grabau  has  suggested  the 
following  varieties  of  limestones  ;  organic  limestones,  such  as  would 
be  afforded  by  the  reef  itself;  coarse,  clastic  limestones  or  calci- 
rudites  (z.  e.,  lime-rubbles) ;  sand  limestones  or  calcarenites  (i.  e.t 


io8  A   HAND  BOOK  OF  ROCKS. 

lime-sands) ;  and  mud  limestones  or  calcilutites  (*.  e.t  lime-muds) 
(Bulletin  Geol.  Soc.  Amer.,  XIV.,  348-352.) 

In  confined  estuaries  of  sea  water  subjected  to  evaporation, 
enough  carbonate  of  lime  is  precipitated  directly  from  solution,  to 
yield  important  strata,  which  are  often  met  in  a  series  of  beds 
associated  with  rock  salt  and  other  precipitated  rocks  as  later  set 
forth.  Calcareous  deposits  from  limy  springs  may  also  almost 
reach  the  dignity  of  rocks,  and  when  abundant  are  called  travertine 
or  calcareous  tufa.  If  particles  of  dust,  etc.,  are  suspended  in  limy 
springs  or  in  concentrated  estuarine  waters,  they  gather  concentric 
shells  of  the  carbonate  and  may  yield  oolitic  deposits  from  the  co- 
alescence of  the  concretions.  Some  algae  likewise  secrete  oolitic 
calcite  and  contribute  extensively  to  rocks. 

Mineral  Composition,  Varieties.  —  Calcite  is  the  chief  mineral  of 
limestones,  and  when  thin  sections  are  magnified  it  exhibits  its 
characteristic  cleavages.  Dolomite  and  siderite  accompany  it  fre- 
quently, and  their  molecules  also  replace  the  calcium  carbonate,  in 
a  greater  or  less  degree,  so  as  to  form  double  carbonates.  An  un- 
broken series  can  readily  be  traced  from  pure  calcium  carbonate, 
through  more  and  more  magnesian  forms,  to  true  dolomite.  Those 
with  over  5  per  cent.  MgO  are  usually  described  as  magnesian 
limestone,  and  when  the  MgO  mounts  well  toward  the  21.72  per 
cent,  in  the  mineral  dolomite,  we  use  the  latter  name.  In  the  same 
way,  a  series  of  ferruginous  varieties  may  be  established  toward  the 
clay  ironstone  and  black  band  ores,  and  a  siliceous  series  toward 
the  flints  and  cherts.  Cherty  limestones  are  a  very  common  variety, 
and  are  referred  to  again  in  connection  with  chert.  When  the 
argillaceous  or  clayey  intermixtures  enter,  argillaceous  or  hydraulic 
varieties  result  that  are  generally  drab  and  close-grained,  and  are 
useful  in  the  manufacture  of  cement.  Bituminous  matter  may  be 
present,  making  the  limestones  black,  and  this,  in  the  form  of 
asphalt,  may  yield  asphaltic  varieties. 

Besides  these  varieties  established  on  the  basis  of  chemical  com- 
position, special  names  may  be  given  because  of  structure.  Thus 
earthy  limestones  tend  to  crumble  to  dirt ;  oolitic  limestones  re- 
semble the  roe  of  a  fish ;  pisolitic  varieties  consist  of  concretions 
of  size  comparable  with  peas  ;  and  other  terms  are  employed,  that 
are  self-explanatory.  Prominent  fossils  suggest  names,  such  as 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         109 

crinoidal,  from  fossil  crinoids ;  coraline,  foraminiferal  and  many 
more  of  local  or  stratigraphic  significance.  Practical  applications 
play  a  part  in  nomenclature,  supplying  "  waterlime,"  "  cement- 
rock,"  "lithographic  limestones,"  etc. 

Metamorphism.  —  Limestones  feel  the  effect  of  metamorphism 
with  exceptional  readiness  and  under  deforming  stresses,  probably 
accompanied  by  elevation  of  temperature,  and  in  the  presence  of 
water,  or  along  the  contacts  with  intruded  dikes  and  sheets  of 
igneous  rocks,  they  lose  their  sedimentary  characteristics,  such  as 
bedding-planes  and  fossils,  and  change  into  crystalline  marbles. 
The  contained  bituminous  matter  becomes  graphite ;  the  alumina 
and  silica  unite  with  the  lime,  magnesia  and  iron  to  give  various 
silicates.  Other  oxides  together  with  the  bituminous  ingredients 
contribute  to  the  various  colorations.  Mechanical  effects  are 
manifested  in  flow  lines,  brecciation  and  other  familiar  features  of 
many  that  are  cut  and  polished  for  ornamental  stones.  Impure 
limestones  which  undergo  these  metamorphic  changes  are  the  most 
prolific  of  all  rocks  in  variety  and  beauty  of  minerals.  Arendal, 
Norway,  and  the  crystalline  limestone  belt  from  Sparta,  N.  J., 
north  through  Franklin  Furnace  are  good  illustrations.  The  crys- 
talline limestones  will  be  again  mentioned  under  the  metamorphic 
rocks. 

Occurrence.  —  Limestones  are  too  common  to  deserve  special 
mention  as  regards  occurrence.  They  are  frequently  met  in  all 
parts  of  the  country,  but  the  Trenton  limestone  of  the  Ordovician, 
the  Niagara  of  the  Silurian  and  the  Subcarboniferous  limestones 
of  the  Mississippi  Valley  are  specially  worthy  of  note. 

III.    REMAINS  OF  ORGANISMS  NOT  LIMESTONES. 

Calcareous  remains  are  much  the  most  important  of  the  contribu- 
tions made  by  organisms  to  rocks,  but  there  are  others,  respectively 
siliceous,  ferruginous  and  carbonaceous,  which  deserve  mention. 

SILICEOUS  ORGANIC  ROCKS. 

The  principal  members  of  this  group  are  infusorial  or  diatoma- 
ceous  earths  ;  siliceous  sinters  ;  and  cherts,  hornstones  or  flints,  the 
three  last  names  being  practically  synonymous.  Infusorial  earths 
consist  of  the  abandoned  frustules  of  diatoms,  which  are  micro- 


i  io  A  HAND  BOOK  OF  ROCKS. 

scopic  organisms  belonging  to  the  vegetable  kingdom.  Though 
not  a  common  rock,  they  yet  are  met  in  series  of  sedimentary 
strata,  both  freshwater  and  marine,  with  sufficient  frequency  to 
justify  their  mention.  Some  foreign  earthy  materials  are  unavoid- 
ably deposited  with  them.  The  siliceous  sinters  are  extracted  from 
hot  springs  by  algae  which,  as  shown  by  W.  H.  Weed,  are  capable 
of  living  and  secreting  silica  in  waters  up  to  1 85  °  F.  They  are  far 
less  important  geologically  than  the  infusorial  earths.  Chert  is  a 
rock  consisting  of  chalcedonic  and  opaline  silica,  one  or  both.  It 
possesses  homogeneous  texture  and  is  usually  associated  with 
limestones,  either  as  entire  beds,  or  as  isolated,  included  masses. 
It  often  has  druses  of  quartz  crystals  in  cavities,  and  in  thin  sections 
under  the  microscope  it  sometimes  exhibits  sponge  spicules. 
Cherts  not  provided  with  these  organic  remains  may  be  regarded 
with  great  reason  as  chemical  precipitates,  and  as  American  varie- 
ties in  the  great  majority  of  cases  lack  them  the  cherts  receive 
more  extended  mention  under  the  chemical  precipitates. 

SiO,  A1,O,       FeaO3          FeO          CaO  MgO         Na,O          K2O          H,O 

Infus.  Earths. 

1.  91-43        2.89  0.66        0.36        0.25        0.63        0.32        3.8 

2.  86.90  4.09  1.26  O.I4  0.51  0.77  0.4!  5.99 

3.  75.86  9.88  2.92  0.29  0.69  0.08  0.02  8.37 

Silic.  Sinter. 

4.  89.54  2.12  tr.          1.71  tr.  1. 12        0.30        5.13 

Chert.                      • . '                       CaCOj.  MgCO,.  • . • 

5-            34-0  0-80                               63.4  1.5                         0.3 

i.  Miocene,  Little  Truckee  River,  Nev.,  R.  W.  Woodward,  4Oth  Parallel  Survey, 
I.,  opposite  p.  542.  2.  Fossil  Hill,  Nev.,  Idem.  3.  Richmond,  Va.,  M.  J.  Cabell, 
Mineral  Resources,  1883-84,  p.  721.  4.  Deposit  from  Old  Faithful,  Yellowstone 
Park,  J.  E.  Whitfield,  for  W.  H.  Weed,  9tb  Ann.  Rep.  Dir.  U.  S.  Geol.  Sur.,  670. 

5.  Cretaceous  chert,  England,  Jukes-Brown  and  Hill,  Quar.  Jour.  Geol.  Soc.,  Aug.,  1889. 

Comments  on  the  Analyses.  —  The  infusorial  earths  are  fairly  high 
in  water,  and  this  is  the  main  cause  of  low  silica,  but,  as  stated 
above,  their  growth  and  accumulation  in  water  make  it  unavoid- 
able that  more  or  less  clay  and  other  sediments  should  mingle 
with  them.  In  these  and  the  other  members  of  the  series,  it  is 
important  to  understand  that  much  of  the  silica  is  opaline,  or  amor- 
phous, hydrated  silica,  and  not  quartz  or  chalcedony.  Tests  of 
the  amounts  soluble  and  insoluble  in  caustic  alkali  are  usually 
made  to  determine  the  proportions  of  the  two,  for,  while  it  is  not 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         in 

an  accurate  separation  —  quartz  and  chalcedony  being  themselves 
somewhat  soluble — it  gives  an  approximate  idea.  No.  4  is  a  de- 
posit separated  from  the  geysers  by  algae  and  evaporation.  No. 
5  is  largely  due  to  sponge  spicules,  mixed  in  with  chalk,  and  there- 
fore is  high  in  calcic  carbonate. 

Mineralogical  Composition,  Varieties.  —  The  mineralogy  of  the 
infusorial  earths  can  be  stated  less  definitely  than  the  chemical  com- 
position. The  individual  diatoms  are  very  minute,  but  the  analyses 
indicate  both  opaline  and  chalcedonic  silica  as  being  present.  In 
the  sinters  and  cherts,  when  the  latter  can  be  shown  to  be  organic, 
the  same  two  varieties  are  recognizable,  and  with  them  are  varying 
amounts  of  calcite.  The  infusorial  earths  are  fine,  powdery  de- 
posits, resembling  white  or  gray,  dried  clays,  but  they  lack  plas- 
ticity and  are  best  recognized  with  the  microscope.  Siliceous  sin- 
ters, often  called  geyserite,  are  cellular  crusts  and  fancifully  shaped 
masses  that  closely  resemble  calcareous  tufas,  but  that  are  readily 
distinguished  by  their  lack  of  effervescence.  Chert  is  dense,  hard 
and  homogeneous,  and  of  white,  gray  or  black  color.  It  readily 
strikes  fire  with  steel,  and  when  it  breaks  has  a  splintery  or  con- 
choidal  fracture.  It  is  often  decomposed  to  powdery  silica  on  the 
outside,  and  in  extreme  cases  may  yield  rather  large  deposits  of 
this  powder,  which  are  called  "tripoli,"  and  are  used  for  various 
practical  purposes.  Mention  may  again  be  made  of  the  cherts 
that  seem  best  explained  by  chemical  precipitation. 

Metamorphism.  —  The  cherts  alone  of  these  rocks  are  of  suffi- 
cient importance  to  attract  attention  in  this  connection,  and  their 
metamorphism  is  briefly  referred  to  on  page  117. 

Occurrence. — Infusorial  earths  are  abundant  near  Richmond,  Va., 
and  on  Chesapeake  Bay,  at  Dunkirk,  and  Pope's  Mills,  Md.  Beds 
deposited  in  evanescent  ponds  or  lakes  are  also  well  known  in 
States  farther  north.  In  the  West,  the  Tertiary  strata  have  yielded 
them  in  Nevada.  In  California  and  Oregon  great  areas  are  re- 
ported by  DiUer.  Siliceous  sinters  produced  by  algae  are  quite 
extensive  in  the  Yellowstone  Park,  and  similar  deposits,  perhaps 
caused  by  the  same  agent,  are  found  in  many  regions  of  hot  springs. 
Sinters  chemically  precipitated  also  occur.  The  most  important 
occurrences  of  chert  are  all  mentioned  together  on  page  118. 


112  A  HAND  BOOK  OF  ROCKS. 

FERRUGINOUS  ORGANIC  ROCKS. 

It  is  a  question  whether  these  deserve  the  dignity  of  rocks,  for 
they  may  with  great  propriety  be  classed  with  the  minerals,  dis- 
tinctively so  called.  It  will  therefore  only  be  mentioned  that 
many  have  attributed  the  formation  of  beds  of  limonite  to  the 
separation  of  iron  hydroxide  by  low  forms  of  organisms.  Even 
granting  this,  it  is  still  true  that  such  limonites  are  insignificant 
when  compared  with  those  that  result  by  purely  inorganic  re- 
actions in  the  decay  of  rocks.  Important  strata  of  cherty  car- 
bonates of  iron  are  present  in  the  iron  mining  districts  around 
Lake  Superior  and  have  been,  no  doubt,  the  principal  source  of 
the  hematites.  Van  Hise  regards  them  as  probably  of  organic 
origin,  but  the  evidence  is  not  decisive  and  they  may  be  chemical 
precipitates.  Clay-ironstone  and  black-band  ores  —  that  is,  argil- 
laceous and  bituminous  ferrous  carbonate  —  sometimes  form  con- 
tinuous beds  instead  of  the  usual  isolated  lenses,  but  when  they  do, 
they  are  not  organic  in  origin,  although  decaying  organic  matter 
may  be  instrumental  in  preserving  the  reducing  conditions  that  are 
necessary  to  the  formation  of  the  ferrous  salt. 

CARBONACEOUS  ORGANIC  ROCKS. 

When  plant  tissue  accumulates  in  damp  places  and  under  a  pro- 
tecting layer  of  water  which  prevents  too  rapid  oxidation,  new 
accessions  may  more  than  compensate  for  loss  by  decay  so  that  ex- 
tensive deposits  may  result.  These  become  progressively  rich  in 
carbon  by  the  loss  of  their  other  elements  and  yield  beds  of  con- 
siderable geological,  but  much  greater  practical  importance.  The 
course  of  the  changes  and  the  several  stages  are  indicated  in  the 
following  table : 

C.  H.  O.  N.  Total. 

Woody  Tissue 50  6  43  i  100 

Peat 59  6  33  2  100 

Lignite 69  5.5  25  0.8  100.3 

Bituminous  Coal 82  5  13  0.8  100.8 

Anthracite 95  2.5  2.5  trace.  100 

The  changes  are  in  the  nature  of  loss  of  oxygen  and  hydrogen, 
and  also  of  carbon,  but  the  decrease  of  the  first  two  is  relatively  so 
much  greater,  that  the  carbon  actually  is  enriched.  The  table  is 
theoretical  in  that  no  account  is  taken  of  the  more  or  less  fortuitous 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         113 

mineral  matter  which  forms  the  ash  together  with  a  small  percentage 
of  incombustibles  in  the  vegetable  tissue  itself.  Peat  is  a  more  or 
less  incoherent  mass  of  twigs  and  stems,  decidedly  carbonized  and 
darkened,  but  with  the  original  structures,  as  a  general  rule,  still 
well  preserved  and  recognizable.  By  gradual  stages  it  passes  into 
lignite,  which  is  still  further  compacted,  and  which  exhibits  the 
original  structures  more  faintly.  In  bituminous  coal,  they  are 
seldom  recognizable,  and  the  aggregate  is  compact  and  black.  In 
anthracite  the  coal  is  dense,  amorphous  and  lustrous.  The  oxi- 
dation necessary  to  the  later  varieties  may  have  been  largely  per- 
formed before  actual  burial  in  other  rocks,  but  the  changes  are 
continuous  and  progressive  in  all. 

Other  organic  derivatives,  such  as  asphalt,  petroleum,  etc.,  are 
not  considered  to  be  of  sufficient  abundance  to  rate  as  rocks. 

Metamorphism. — Anthracite  is  locally  produced  from  bituminous 
coal  near  igneous  intrusions,  and  by  regional  metamorphism,  as 
later  explained.  The  chemical  changes  are  the  same  as  those 
progressive  ones  above  outlined,  but  are  doubtless  more  rapidly 
brought  about.  Anthracites  become  graphitic,  and,  as  a  theoret- 
ical extreme,  pass  into  graphite.  Natural  cokes  are  also  produced 
along  intruded  dikes. 

Occurrence.  —  Peat  favors  cool  and  moist  latitudes  in  all  parts  of 
the  world,  and  is  chiefly  of  fresh  water  origin.  Lignites  and  coals 
are  best  developed  in  the  Carboniferous  and  Cretaceous  strata,  and 
where  the  former  occur  in  the  East  and  the  latter  in  the  West, 
they  often  contain  coal  seams. 

IV.    PRECIPITATES  FROM  SOLUTION. 

The  name  of  this  group  indicates  the  character  of  the  rocks  that 
comprise  it.  Bearing  in  mind  the  condition  established  at  the 
outset,  p.  I,  that  a  rock  should  form  an  essential  part  of  the  earth, 
it  is  evident  that  water  is  the  only  natural  solvent  abundant  enough 
to  yield  such  rocks,  and  that  only  the  most  widespread  compounds 
which  are  notably  soluble  in  it,  or  in  its  common  solutions  of  other 
more  soluble  salts,  can  meet  this  requirement.  The  rocks  may  be 
conveniently  taken  up  under  the  following  heads.  I.  Precipitates 
involving  the  alkaline  earths  and  alkalies.  2.  Siliceous  precipitates. 
3.  Ferruginous  precipitates. 


U4  A   HAND  BOOK  OF  ROCKS. 

PRECIPITATES  INVOLVING  THE  ALKALINE  EARTHS  AND  ALKALIES. 

The  carbonate  of  lime  in  stalactites,  stalagmites  and  crusts  on 
the  walls  and  floors  of  caves  in  limestone  or  in  the  surface  deposits 
from  limy  springs,  affords  a  rock  of  this  character.  It  is  a  form 
of  limestone,  from  pure  varieties  of  which  it  does  not  differ  in  com- 
position, although  its  banded  structure  and  rings  of  growth,  which 
we  may  describe  by  Posepny's  useful  word  "  crustification,"  in  a 
measure  distinguish  it.  Naturally  such  deposits  are  often  beauti- 
fully crystalline,  free  from  admixture  except  of  associated  dissolved 
materials  and  as  a  rule  purer  than  sedimentary  limestones.  They 
yield  our  well-known  onyx  marbles.  Some  regularly  stratified  de- 
posits of  limestones  that  are  associated  with  the  precipitated  rocks 
next  discussed  have  doubtless  originated  together  with  the  latter. 

Gypsum  and  rock  salt  are  the  chief  members  of  this  subgroup. 
They  occur  quite  invariably  in  association,  and  have  resulted  alike 
from  the  evaporation  of  sea-water  and  from  the  drying  up  of  lakes, 
originally  fresh.  Both  are  mixed  more  or  less  with  dust  and  other 
mechanical  sediments  washed  or  blown  into  the  evaporating  res- 
ervoir, or  are  interbedded  with  other  salts  which  were  present  in  a 
minor  capacity  in  the  mother  liquor,  but  instances  of  thick  beds, 
especially  of  rock  salt  of  surprising  purity,  are  well  known.  When 
these  attain  several  hundred  or  even  a  thousand  feet,  it  is  evident 
that  more  than  twenty-five  times  this  depth  of  salt  water,  on  the 
basis  of  the  known  composition  of  the  sea,  would  have  to  be  evap- 
orated, and  this  is  a  practical  absurdity  for  any  conceivable  con- 
fined body,  even  with  occasional  renewals  from  breaches  of  the  bar- 
rier. It  would  be  necessary  to  assume  wide  stretches  of  shallows 
which  were  practically  evaporated  to  dryness,  while  at  the  same  time 
subsidence  of  the  coast  was  progressing  at  just  about  the  necessary 
rate  to  keep  pace  with  the  growth  of  the  salt.  The  recent  ex- 
planation, however,  advanced  as  the  "  Bar  theory,"  by  Ochsenius,* 
clears  it  up.  We  need  only  to  assume  a  relatively  deep  and  nearly 
land-locked  estuary,  with  a  shallow  bar  between  it  and  the  sea. 
Evaporation  continually  concentrates  the  confined  salt  water  and 
especially  the  portion  on  the  shallow  bar.  This,  becoming  rich 
in  mineral  matter  and  of  high  specific  gravity,  flows  inward  and 

*  Zeitschrift  f.  Praktische  Geologic,  May  and  June,  1893.  An  excellent  abstract  by 
L.  L.  Hubbard  appears  in  the  Geol.  of  Michigan,  V.,  Part  II.,  p.  ix. 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         115 

down  the  slope  of  the  bar  to  the  bottom  of  the  estuary.  In  the 
course  of  time,  and  allowing  for  the  influence  of  pressure  in  the 
depths  and  of  temperature,  conditions  favorable  to  precipitation, 
first,  of  the  insoluble  gypsum,  later  of  the  more  soluble  common 
salt  will  be  reached,  and  in  varying  and  alternating  layers  they  will 
be  built  up  indefinitely,  or  until  some  upheaval  or  subsidence  alters 
the  relations  of  the  estuary  to  the  sea.  More  or  less  anhydrite  is 
also  deposited,  and  is  later  found  in  extended  cross-sections  of 
salt-bearing  strata.  The  most  soluble  ingredients,  such  as  KC1, 
MgCl2,  MgSO4,  etc.,  become  continually  richer  in  the  mother 
liquor,  and  unless  this  is  also  finally  evaporated,  they  escape  and 
are  not  found  in  the  series.  So  far  as  we  know,  the  Stassfurt  dis- 
trict, in  Germany,  is  almost  the  only  place  where  this  escape  has 
been  prevented  on  a  large  scale,  although  rock  salt  is  of  world- 
wide distribution. 

Gypsum  forms  at  times  gray  or  black  earthy  beds,  that  look  very 
much  like  limestone,  but  of  course  do  not  effervesce.  Again,  it  is 
in  white,  cream-colored  or  more  deeply  tinted  layers,  yielding  ala- 
baster. Minor  portions  are  in  condition  of  selenite,  the  clear,  trans- 
parent variety,  and  thin  coats  of  native  sulphur  are  seldom  lacking. 
Rock  salt  forms  crystalline  beds,  often  stained  red  or  brown,  by  iron 
oxide.  Both  gypsum  and  salt  may  impregnate  associated  sediments 
more  or  less,  yielding  gypseous  or  saline  shales  and  marls.  In 
many  localities  gypsum  deposits  have  undergone  a  complex  series 
of  chemical  changes  in  the  general  nature  of  deoxidation  from  car- 
bonaceous matter  present,  so  as  to  yield  native  sulphur  in  large 
amounts. 

Metamorphism,  —  None  of  the  above  rocks  are  worthy  of  men- 
tion as  regards  metamorphism. 

Occurrence,  —  In  America,  gypsum  is  found  especially  in  the 
Upper  Silurian  of  New  York  ;  the  Lower  Carboniferous  of  Michi- 
gan and  Nova  Scotia ;  the  Triassic  in  the  states  of  the  Great  Plains 
such  as  Kansas  and  Texas ;  in  undetermined  Mesozoic  in  Iowa ; 
and  in  the  Jura-Trias  or  in  undetermined  strata  in  Colorado,  Utah 
and  the  West.  Rock  salt  occurs  in  the  Upper  Silurian  of  southern 
New  York ;  in  the  Triassic  of  Kansas ;  in  the  Quaternary  (?)  of 
Petite  Anse,  La.,  and  at  many  places  of  recent  geological  age  in 
the  West. 


n6  A   HAND  BOOK  OF  ROCKS. 

SILICEOUS  PRECIPITATES. 


(a)SiO,(b)SiO, 

A18O,   FesO, 

CaO 

MgO 

KaO 

Na,O 

£oss. 

Sp.Gr. 

Geyserite. 

I.          81.95 

6.49       tr. 

0.56 

0.15 

0.65 

2.56 

7.50 

Cherts. 

2.          99.46 

0.29 

0.4 

tr. 

0-34 

3-       3-35  95-78 

0.16 

tr. 

O.OI 

0.20 

4.       4.52  93.65 

0.83 

0.05 

O.OI 

0.78 

5-          98.io 

0.24  0.27 

0.18 

0.23 

1.16 

6.          94.91 

2.85 

0.42 

tr. 

Sil.  OSlite. 

7-          95.83 

2.03 

1-93 

tr. 

2.63 

CaCO, 

MgCO, 

8.          56.50 

1.50 

16.84 

2.60 

12-54 

2.688 

9-           3-70 

1.42 

88.71 

8.09 

2.654 

Cherty  iron  carbonates. 

CaO 

MgO 

FeO 

MnO 

CO, 

10.           58.23 

0.06  5.01 

0.38 

9-59 

18.41 

0.25 

2.08 

5.22 

II.          46.46 

0.240.64 

1.87 

3-io 

26.28 

O.2I 

1.22 

19.96 

12.             28.86 

1.29  i.oi 

0-74 

3.64 

37-37 

0.97 

0.68 

25.21 

NOTE,     (a)  SiO2  means  silica  soluble  in  caustic  alkali  ; 

(b)  SiO,  silica  insoluble  in 

the  same. 

I.  Geyserite,  Splendid  Geyser,  Yellowstone  Park,  J.  E.  Whitfield  for  W.  H.  Weed, 
9th  Ann.  Rep.  U.  S.  Geol.  Sur.,  670.  2.  Gray  unaltered  chert,  Joplin,  Mo.  Anal- 
ysis made  by  U.  S.  Geol.  Surv.  Quoted  in  Ann.  Rep.  Geol.  Sur.  Ark.,  1896,  III., 
161.  3.  White  altered  chert,  Galena,  Kan.,  Idem.  4.  Unaltered  chert,  Bellville,  Mo., 
Idem.  5.  Decomposed  chert,  or  Tripoli,  Seneca,  Mo.,  W.  H.  Seamon.  Quoted  by 
E.  O.  Hovey,  Amer.  Jour.  Sci.,  Nov.,  1894,  406.  6.  Chert,  Roaring  Springs,  Newton 
Co.,  Mo.,  J.  D.  Robertson,  for  E.  O.  Hovey,  Idem.  7.  Siliceous  oolite,  Center  Co., 
Penn.,  Barbour  and  Torrey,  Amer.  Jour.  Sci.,  Sept.,  1890,  249.  8.  Silica-lime  oolite, 
Idem.  9.  Lime-silica  oolite  on  same  specimen  as  No.  8,  Idem.  10,  n.  Cherty  iron 
carbonates,  N.  E.  Minn.,  T.  M.  Chatard,  for  C.  R.  Van  Hise,  Monograph  XIX., 
U.  S.  Geol.  Survey,  192.  12.  Cherty  iron  carbonate,  Sunday  Lake,  Gogebic  Range, 
Mich.,  W.  F.  Hillebrand,  Idem. 

Comments  on  the  Analyses.  —  The  first  seven  are  high  in  silica, 
some  approximating  chemical  purity.  No.  I  has  admixtures  of 
mud  thrown  out  by  the  geyser  from  its  walls.  The  five  cherts, 
2-6  inclusive,  have  but  slight  amounts  of  alumina,  iron  and  lime, 
and  low  percentages  of  water.  Nos.  3  and  4,  by  the  determina- 
tions of  soluble  silica  give  us  some  idea  of  the  amount  of  the 
opaline  form  that  is  present.  The  three  analyses  7,  8  and  9  are 
a  most  instructive  series,  passing  as  they  do  from  nearly  pure  silica 
into  a  moderately  siliceous,  magnesian  limestone,  from  which  the 
first  two  are  thought  to  have  been  derived  by  replacement.  Nos. 
10,  1 1  and  1 2  are  the  curious  cherty  carbonates  of  iron  from  which 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         117 

the  Lake  Superior  iron  ores  have  been  formed  by  subaerial  decay. 
Their  richness  in  magnesia  as  compared  with  lime  is  noteworthy. 

Mineralogical  Composition,  Varieties.  —  Cherts  are  so  exceed- 
ingly fine  grained  that  they  give  no  indication  of  their  constituent 
minerals  to  the  unaided  eye.  The  microscope  shows,  however, 
that  they  are  chiefly  chalcedony  in  excessively  minute  crystals, 
with  which  are  associated  varying  amounts  of  opaline  silica,  quartz 
crystals,  calcite  or  dolomite  rhombs  and  dusty  particles  of  iron 
oxide.  In  foreign  cherts  as  stated  above  on  p.  102,  sponge  spic- 
ules  have  been  met,  but  not  in  the  important  American  varieties. 
Cherts  often  have  an  outer  powdery  crust,  due  to  alteration,  and 
while  as  shown  by  analysis  5,  this  may  not  mean  any  notable 
chemical  change,  it  may  penetrate  whole  beds  and  leave  only  a 
white,  incoherent  mass  called  "  tripoli,"  that  is  used  for  a  polishing 
powder  and  for  various  other  purposes.  Cherts  have  spherulites 
occasionally  and  are  still  more  often  oolitic.  The  cherty  or  sili- 
ceous rocks  of  the  formations  containing  the  Lake  Superior  iron  ores 
are  mixtures  of  chalcedonic  silica  and  carbonate  of  iron  in  varying 
proportions,  and  in  their  alteration  they  afford  more  or  less  sharply 
differentiated  jaspers  and  hematites.  Three  analyses  of  varying 
composition  are  given  above,  Nos.  10,  n  and  12.  Cherts  vary  in 
color  from  black  through  gray  to  creamy  white. 

As  stated  earlier,  cherts  are  intermingled  in  all  proportions  with 
limestones.  They  are  very  puzzling  problems  as  regards  origin. 
Where  devoid  of  organisms,  the  majority  of  observers  regard  them 
as  in  some  way  precipitated  chemically  from  sea  water,  possibly  as 
gelatinous  silica.  They  may  also  result  by  replacement  of  limestone. 
Their  structure  and  relations  give  us  few  definite  clues  on  which  to 
base  a  firm  conclusion.  As  earlier  stated,  others  regard  them  as 
derived  from  siliceous  remains  of  organisms,such  as  sponges,  radio- 
larians  and  the  like,  which  may  have  been  redissolved  and  worked 
over  into  chalcedony,  making  them  practically  precipitates.  Cherts 
are  also  called  hornstone  and  flint. 

Metamorphism.  —  Purely  siliceous  cherts  are  unpromising  sub- 
jects for  metamorphism,  except  as  they  yield  silica  for  the  produc- 
tion of  silicates  from  cherty  limestones.  The  ferruginous  cherts 
of  Lake  Superior  pass  into  actinolitic  and  magnetitic  slates,  a  most 
interesting  change,  especially  in  the  former  case.  The  lime,  mae- 


Ii8  A  HAND  BOOK  OF  ROCKS. 

nesia  and  iron  are  combined  with  silica  under  the  metamorphosing 
influences  so  as  to  yield  the  variety  of  actinolite  called  griinerite. 

Occurrence.  —  The  abundance  of  cherts  or  related  rocks  in  the 
region  of  Lake  Superior,  either  associated  with  limestone  or  in  the 
cherty  carbonates  described  above,  is  remarkable.  In  their  eco- 
nomic products,  they  are  the  most  important  strata  present.  The 
Siluro- Cambrian  limestones  are  often  cherty  both  east  and  west, 
and  in  the  New  York  and  Ohio  Devonian,  the  so-called  "  Cornifer- 
ous  "  limestone  was  named  from  its  richness  in  "hornstone."  In 
the  Mississippi  Valley  the  lower  Carboniferous  strata  are  particu- 
larly prolific  in  cherts.  Fractured  cherts  are  the  chief  gangue  of 
the  zinc  ores  of  southwest  Missouri. 

FERRUGINOUS  PRECIPITATES. 

Some  iron  ores  doubtless  originate  in  this  way,  and  the  processes 
by  which  the  soluble  proto-salts  are  oxidized  and  precipitated  as 
the  insoluble  ferric  hydroxide  are  well  understood.  But  they  may 
be  considered  rather  as  minerals  than  as  rocks.  The  cherty  iron 
carbonates  of  the  preceding  section  have  already  been  cited,  and 
the  clay  ironstones  and  black-band  iron  ores  are  omitted  from 
further  mention  for  the  same  reasons  as  are  the  limonites. 

THE  DETERMINATION  OF  THE  AQUEOUS  AND  EOLIAN  ROCKS. 

The  members  of  this  series  are  much  easier  to  recognize  than 
are  the  igneous.  Breccias,  conglomerates  and  sandstones  are  at 
once  apparent  from  their  fragmental  character.  Breccias  differ 
from  conglomerates  in  the  angular  shape  of  their  component  frag- 
ments. As  the  sandstones  become  finer,  the  argillaceous  varieties 
may  be  distinguished  by  the  peculiar  odors  emitted  by  all  clays  and 
clayey  rocks  when  breathed  upon.  The  calcareous  sandstones 
and  marls  betray  themselves  by  effervescence  with  acid.  All  lime- 
stones, unless  too  rich  in  magnesia,  effervesce  in  cold  acid,  and  the 
more  readily  if  first  scraped  up  into  a  little  heap  of  powder  with  a 
knife.  Dolomites  effervesce  much  less  readily,  and  warm  acid  may 
be  necessary.  Infusorial  earth  may  need  the  microscope  for  its 
certain  identification,  and  then  the  abundance  of  the  little  organisms 
is  very  apparent.  The  cherts  are  so  characteristic  in  appearance 
as  to  admit  of  little  uncertainty,  except  as  compared  with  the  silici- 


THE  AQUEOUS  AND  EOLIAN  ROCKS.         119 

fied  tuffs  and  excessively  fine  felsites,  called  petrosilex,  in  which  case 
geological  surroundings  or  the  microscope  are  the  only  resources. 
The  ferruginous  rocks,  if  such  be  allowed,  are  self-evident,  as  are  the 
carbonaceous.  Gypsum  is  easily  recognized  when  in  the  crystal- 
line form,  but  when  black  and  earthy,  the  observer  may  be  forced 
to  determine  its  lack  of  effervescence,  and  to  make  a  sulphur  test 
with  the  blowpipe.  Nevertheless  with  these  rocks  as  with  the 
igneous,  although  to  a  less  degree,  it  is  very  advisable  to  gain  ex- 
perience with  correctly  labeled  study  collections  or  with  the  syste- 
matic exhibits  of  a  museum,  so  that  the  student  may  have  a  fund 
of  personal  observation  back  of  him  from  which  to  draw,  and  0? 
which  to  depend  when  a  rock  comes  up  for  determination. 

For  field  work  and  travel,  it  is  well  to  appreciate  that  a  few  dry 
crystals  of  citric  acid,  that  can  be  dissolved  in  a  little  water  as 
needed,  serve  very  well  for  tests  of  effervescence.  They  are  more 
safely  carried  than  are  liquid  mineral  acids. 

TABLE  OF  CLASSIFICATION. 

On  p.  1 20  a  summarized  table  of  classification  of  the  breccias 
and  the  sedimentary  rocks  proper  is  given  which  will  present  a 
birds-eye-view  of  the  varieties  just  described.  It  may  be  compared 
with  the  similar  one  of  the  metamorphic  rocks  on  p.  160  since  so 
many  of  the  latter  are  derived  from  the  sediments. 


I2O 


A   HAND  BOOK  OF  ROCKS. 


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CHAPTER  IX. 

THE  METAMORPHIC  ROCKS.      INTRODUCTION.     THE  ROCKS  PRO- 
DUCED BY  CONTACT  METAMORPHISM. 

The  word  metamorphism  was  first  introduced  into  geological 
literature  by  Lyell  in  1832,  and  was  used  to  describe  the  processes 
by  which  rocks  undergo  alteration.  It  was  particularly  applied  by 
him  to  those  stratified  rocks  that,  from  deep  burial  in  the  earth, 
and  from  the  consequent  heat  and  pressure  to  which  they  have 
been  subjected,  have  assumed  structures  and  textures  resembling 
those  of  the  unstratified  primary  or  plutonic.  In  this  sense  it  has 
been  generally  employed  since,  and  it  implies  an  increase  in  crys- 
tallization, hardness  and  those  attributes,  which  are  especially 
associated  with  the  crystalline  schists,  as  contrasted  with  the  un- 
altered sediments. 

The  literal  meaning  of  the  phrase  "  the  processes  by  which  rocks 
undergo  alteration  "  may,  nevertheless,  be  somewhat  more  com- 
prehensive than  this,  and  may  be  made  to  include  the  changes 
produced  by  atmospheric  agents,  which  we  ordinarily  describe  by 
the  term  weathering,  and  in  the  following  pages  the  products  of 
this  latter  form  of  alteration  will  be  briefly  considered  as  a  third 
and  concluding  group. 

The  metamorphic  rocks  will  therefore  be  taken  up  under  the 
following  three  classes  : 

I.  Rocks  reduced  by  Contact  Metamorphism. 
II.  Rocks  produced  by  Regional  Metamorphism. 

III.  Rocks  produced  by  Atmospheric  Weathering. 

By  contact  metamorphism  is  meant  the  series  of  changes  that 
are  effected  by  an  igneous  intrusion,  such  as  a  dike  or  a  laccolith 
upon  the  rocks  through  which  it  is  intruded.  These  changes  are 
often  profound,  and  are  brought  about  by  the  heat  of  the  intrusion 
as  well  as  by  vapors  and  hot  solutions  which  it  may  likewise  give 
forth.  The  wall-rock  may  be  itself  igneous  or  sedimentary,  or 
even  metamorphic.  This  form  of  metamorphism  is  sometimes 
called  "local"  as  contrasted  with  "regional." 

121 


122  A  HAND  BOOK  OF  ROCKS. 

By  regional  metamorphism  we  describe  the  series  of  changes 
which  are  produced  in  the  rocks  of  wide  areas  or  "  regions  "  by 
deep  burial,  mountain-making  upheavals,  and  by  heat  and  pres- 
sure. Although  Lyell  had  stratified  rocks  before  him  as  the  chief 
materials  on  which  these  agents  acted,  yet  it  is  well  recognized  to- 
day that  igneous  rocks  are  no  less  profoundly  affected,  and  indeed 
that  the  results  of  their  alteration  may  be  almost  or  quite  indistin- 
guishable from  those  derived  from  sediments.  But  there  is  great 
uncertainty  as  to  the  original  condition  of  many  regionally  meta- 
morphosed rocks,  and  although  the  endeavor  has  been  made  in 
previous  pages  to  throw  as  much  light  on  them  as  possible,  by 
systematically  referring  to  the  alteration  and  metamorphism  of 
simple  types,  nevertheless,  many  are  obscure,  and  in  their  history 
are  involved  some  of  the  profoundest  problems  of  geology. 

By  atmospheric  weathering  is  meant  the  series  of  changes  wrought 
in  rocks  at  or  near  the  surface  of  the  earth,  by  the  ordinary  atmos- 
pheric agents,  water,  oxygen,  carbonic  acid  and  the  like.  The  changes 
are  chiefly  in  the  nature  of  disintegration,  loss  of  soluble  ingredients 
and  decomposition,  and  in  general  they  produce  a  marked  shrink- 
age of  bulk. 

It  is  important  to  appreciate  that  under  whatever  form  the  meta- 
morphic  rocks  are  met,  they  are  of  necessity  alteration  products  of 
the  two  grand  divisions  over  which  we  have  already  passed. 

GENERALITIES  REGARDING  CONTACT  METAMORPHISM. 
Widening  observation  has  shown  that  contact  metamorphism  is 
produced  by  all  varieties  of  igneous  rocks  and  that  it  may  be 
broadly  stated  to  be  independent  of  the  kind  of  rock  forming  the 
intrusion.  Granites,  syenites,  nephelite-syenites,  diorites,  gabbros 
and  even  peridotites  have  in  one  place  and  another  proved  to  be 
efficient  agents.  Yet  the  following  statements  may  be  said  to 
hold  good. 

1.  Plutonic  rocks  are  more  favorable  to  it  than  volcanic.     This 
follows  because  plutonic  rocks  cool  slowly  at  considerable  depths 
and  stand  therefore  at  high  temperatures  for  long  periods  next 
their  walls. 

2.  Magmas  rich  in  mineralizers  are  much  more  favorable  than 
are  those  poor  in  them.    This  naturally  follows  from  the  powerful 


THE  METAMORPHIC  ROCKS.  123 

influence  exerted  by  escaping  vapors.  It  is  tantamount  to  saying 
that  acidic  rocks  are  in  general  more  efficient  than  basic  ones, 
because  experiment  shows,  and  field  observation  indicates,  that 
abundant  absorbed  vapors  accompany  and  facilitate  the  fusion  of 
the  rocks  high  in  silica,  whereas  basic  rocks  are  much  more  largely 
the  results  of  dry  fusion.  Granites,  for  instance,  are  the  com- 
monest and  most  effective  agents  of  contact  metamorphism. 

3.  As  regards  the  walls,  sedimentary  rocks  possess  varying  sus- 
ceptibilities.    Highly  siliceous  sandstones  and  conglomerates,  for 
example,  are  stubborn  subjects,  and  manifest  but  slight  altera- 
tion ;  but  highly  aluminous  or  calcareous  beds  are  favorable  to 
recrystallization,  because  they  contain  the  alumina,   iron,   lime, 
magnesia  and  the  alkalies  which  will  combine  with  silica,  under 
metamorphosing  influences,  to  yield  copious  contact  minerals.    Of 
all  rocks,  impure  limestones  yield  the  most  varied  and  interesting 
results. 

4.  With  a  favorable  intrusion,  the  apparent  distance  to  which 
the  metamorphosing  influence  penetrates,  depends  on  the  angle  of 
emergence  of  the  intrusion.     If  it  comes  up  at  a  low  angle  it  may 
lie  but  a  short  distance  below  the  surface  for  a  considerable  stretch 
on  one  side  of  the  outcrop,  so  that  the  metamorphosed  area  may 
apparently  extend  to  a  great  distance,  although  at  no  point  far 
from  the  source  of  heat.     Around  a  vertical  dike  the  distance  would 
naturally   be   less.     Again,  the   alterations   progress   much   less 
readily  across  the  bedding  of  stratified  rocks  than  along  it.    Hence, 
an  intrusion  that  cuts  across  the  bedding  produces  more  wide- 
spread effects  than  does  one  parallel  with  it. 

5.  It  is  believed  by  many,  especially  among  English  and  Ger- 
man observers,  that  there  is  very  slight  migration  of  material  dur- 
ing metamorphism,  and  therefore  that  the  contact  minerals  have 
resulted  from  the  silica  and  the  bases  which  were  practically  in  the 
same  places  before  the  intrusion  as  after  it.     It  follows  that  there 
has  been  no  chemical  introduction  or  substitution,  but  only  rear- 
rangement of  molecules  during  the  process.     An  analysis,  there- 
fore, of  a  reasonably  large-sized  sample  would  indicate  the  compo- 
sition of  the  original  rock,  except  so  far  as  water,  carbonic  acid 
and  other  volatile  ingredients  have  been  driven  off.     From  obser- 
vations upon  an  intrusion  of  granite  in  Westmoreland,  England, 


124  A  HAND  BOOK  OF  ROCKS. 

which  cuts  a  decomposed,  basic,  amygdaloidal  lava,  Alfred  Harker 
concluded  that  the  migration  had  not  exceeded  one  twentieth  of 
an  inch.  But  among  the  French  much  greater  power  of  chemically 
affecting  the  walls  is  attributed  to  intrusions,  and  in  instances  it 
certainly  seems  as  if,  in  addition  to  the  fluorine  and  boron  which  we 
all  know  penetrate  into  wall  rocks  during  the  escape  of  mineralizers, 
hydro-fluosilicic  acid  might  impart  silica  and  that  some  of  the  bases, 
and  especially  the  alkalies,  might  migrate  in  heated  solutions,  to  a 
moderate  distance. 

The  soundness  of  the  belief  in  the  migration  of  material  from 
the  intrusive  mass  has  been  demonstrated  beyond  question  in  later 
years  by  the  study  of  the  garnet  zones  produced  from  limestones 
by  moderately  or  decidedly  acidic  igneous  rocks.  The  occasional 
appearance  of  copper  ores  in  the  zones  has  given  the  problem 
technical  as  well  as  purely  scientific  importance.  There  are  two 
points  of  especial  significance  which  have  been  developed.  For 
years  it  was  assumed  that  the  variety  of  garnet  present  was  gros- 
sularite,  3CaO,Al2O3,3SiO2.  Since  alumina  in  the  form  of  kao- 
linite,  and  silica,  as  comminuted  quartz,  chalcedony,  chert,  etc., 
are  common  and  widespread  ingredients  in  limestone,  the  conclu- 
sion was  reached  that  under  the  influence  of  heat  from  the  intru- 
sive mass,  the  kaolinite  and  quartz  or  other  forms  of  silica,  had 
combined  with  the  calcite,  driving  off  the  carbon  dioxide  and  pro- 
ducing grossularite  as  the  result  Chemical  analyses,  which  are 
now  available  from  many  localities,  have  shown  however  that  the 
garnet  is  grossularite  only  in  small  part,  but  that  it  is  usually 
or  predominantly  andradite,  3CaO,Fe2O3,3SiO2,  a  variety  which 
cannot  be  distinguished  from  grossularite  with  the  eye.  Ferric 
oxide  is  not  found  in  the  limestones  in  anything  like  the  amount 
called  for  and  it  has  therefore  been  necessary  to  derive  it  from  the 
igneous  rock.  This  first  point  is  corroborated  by  a  second  as  fol- 
lows. The  contact  zones  from  limestones  are  often  provided 
with  great  bodies  of  magnetite  or  specular  hematite,  which  observers 
have  been  unable  to  explain  satisfactorily  in  any  other  way  than 
by  emissions  from  the  igneous  mass  in  its  cooling  stages.  The 
Iron  Springs  district,  Utah,  furnishes  a  good  illustration,  but  there 
are  many  more  in  the  southwestern  United  States  and  in  Mexico. 
Many  geologists  have  thus  come  to  believe  that  an  intrusive  mass 


THE  METAMORPHIC  ROCKS.  125 

in  its  cooling  stages  emits  water,  gas,  and  probably  other  vapors, 
and  with  them  silica,  iron,  often  copper  and  very  likely  alumina. 
While  garnet  (andradite)  is  commonest,  diopside,  vesuvianite, 
epidote,  wollastonite  and  some  rarer  minerals  are  frequent  associates. 
Copper  ores  with  small  gold  values  are  more  frequently  associated 
than  other  ores,  save  those  of  iron. 

The  ore-bodies  are  of  irregular  shape  and  distribution.  The 
garnet  and  related  lime-silicates  were  the  first  minerals  to  form  and 
were  followed  by  the  magnetite,  pyrite  and  chalcopyrite. 

6.  Notwithstanding  the  truth  of  the  foregoing  generalities,  it  is 
a  curious  fact  that  contact  effects  are  sometimes  strangely  lacking 
where  we  would  naturally  expect  them,  and  they  are  often  of 
varying  intensity  and  irregular  distribution,  where  they  do  occur. 
These  anomalies  can  in  part  be  explained  by  the  general  principles 
already  cited,  of  which  no  doubt  the  presence  or  absence  of  min- 
eralizers,  the  superheated  or  relatively  cold  condition  of  the  intru- 
sion are  chief.  But  every  observer  of  wide  experience  is  some- 
times much  puzzled  by  what  he  meets  in  Nature. 

I.   THE  ROCKS  PRODUCED  BY  CONTACT  METAMORPHISM. 

Although  the  principal  results  of  contact  metamorphism  are 
manifested  in  the  walls  of  the  intrusion,  the  igneous  rock  is  itself 
influenced.  It  is  therefore  necessary  to  note  both  the  internal  and 
the  external  effects,  or  those  upon  the  intrusion  and  those  upon  the 
walls.  The  area  over  which  the  latter  are  manifested  is  often 
called  the  aureole,  and  the  concentric  rings  of  decreasing  alteration 
as  one  passes  outward  from  the  intrusion  are  called  zones. 

Internal  Effects.  —  The  igneous  rock  suffers  a  relatively  rapid 
loss  of  heat  in  its  marginal  portions  as  compared  with  its  interior, 
and  as  a  result  it  very  commonly  assumes  a  porphyritic,  felsitic  or 
even,  just  as  the  contact,  a  glassy  texture,  although  it  may  be 
granitoid  within.  Where  these  textures  are  well  developed  the 
passage  from  one  to  the  other  is  extremely  gradual,  and  if  the  wall 
rock  has  been  originally  a  shale  or  a  clay  that  has  been  baked  to  a 
dense  mass,  one  may  need  microscopic  examination  to  determine 
where  the  intrusion  ends  and  the  wall  rock  begins.  The  changes 
in  texture  in  the  intrusion  are  accompanied  more  or  less  by 
changes  in  chemical  composition  and  in  not  a  few  cases  progres- 


126  A  HAND  BOOK  OF  ROCKS. 

sive  analyses  have  shown  the  margins  to  be  much  more  basic  than 
the  interior  of  the  intrusion.  The  chilling  of  the  former  has  thus 
produced  chemical  rearrangements  in  the  magma  previous  to  con- 
solidation. 

External  Effects.  —  Recalling  the  statement  earlier  made  that 
within  the  limits  already  set  forth  the  character  of  the  intrusion  is 
immaterial,  the  most  convenient  and  intelligible  method  of  treat- 
ment will  be  to  briefly  outline  several  typical  cases  wherein  the 
commoner  sedimentary  rocks  are  known  to  have  been  affected, 
and  then  to  refer  to  one  or  two  instances  wherein  igneous  or 
regionally  metamorphic  ones  have  suffered  alteration.  The  same 
order  will  be  preserved  for  the  sediments  as  appears  under  Chapters 
VII.  and  VIII. 

Breccias  are  too  limited  in  distribution  to  be  a  serious  factor. 
Conglomerates  and  sandstones  so  generally  consist  of  silica,  that 
they  supply  but  little  raw  materials  of  a  favorable  kind.  The  small 
amounts  of  alumina  present  may  combine  with  the  silica  to  afford 
sillimanite  (A12O3  SiO2)  and  stimulated  circulations  of  hot  water 
may  cause  added  deposition  of  quartz  around  the  grains  so  as  to 
develop  increased  hardness. 

With  shales  and  clay  rocks,  even  if  in  the  form  of  slate  (see  later, 
p.  145),  the  effects  are  more  pronounced ;  and  around  intrusions 
in  them  well-marked  and  well -identified  zones  have  been  described. 

At  the  contact  of  the  igneous  rock  with  the  sediment  a  breccia 
or  "mixed  zone"  of  intrusive  and  fragments  of  wall-rock  is  some- 
times, although  not  always,  met.  More  commonly  the  shales,  slates, 
clay  or  their  kindred  rocks  are  baked  and  altered  to  a  dense  flinty 
product  known  as  a  hornfels  or  hornstone,  which  latter  name  in 
this  sense  is,  however,  not  to  be  confused  with  its  use  for  flints  and 
cherts.  It  breaks  in  irregular,  angular  masses  and  has  a  very 
close  resemblance  to  dense  trap.  Its  mineralogy  is,  as  a  general 
thing,  a  subject  for  microscopic  study,  but  it  may  be  said  that 
biotite  in  small  scales  is  rather  the  most  widespread  mineral  present, 
and  that  andalusite,  garnet,  cyanite,  staurolite,  tourmaline,  ottrelite, 
rutile,  hornblende,  feldspars  and  other  minerals  more  or  less  char- 
acteristic of  such  surroundings  frequently  appear.  They  may  be 
of  considerable  size  and  the  prisms  of  andalusite  of  the  variety 
chiastolite,  with  the  light  and  dark  maltese  crosses  showing  in  their 


THE  METAMORPHIC  ROCKS.  127 

cross-sections,  are  especially  frequent  As  the  contact  is  left  the 
hornfels  often  passes  into  mica  schist.  Farther  out  the  mineralog- 
ical  changes  become  less  marked ;  the  andalusite  and  other  crystals 
are  less  and  less  well  developed  and  finally  shade  into  mere  dark 
spots  or  aggregates  of  biotite,  magnetite  and  bituminous  matter. 
When  even  these  fade  out  the  unchanged  sediment  is  met.  In 
some  localities  it  has  therefore  been  possible  to  establish  three 
zones,  which  are,  in  the  reverse  order  of  the  above  succession,  the 
knotty  or  spotted  slates,  the  knotty  mica  schists,  and  the  hornfels, 
usually  with  andalusite.  By  knotty  is  meant  the  aspect  given  by 
the  larger  contact  minerals  in  the  midst  of  finer  aggregates.  These 
are  the  names  adopted  for  a  well-known  contact  studied  by  the  emi- 
nent German  petrographer,  Rosenbusch,  in  the  Vosges  Mountains. 
At  a  famous  American  locality  in  the  Crawford  Notch  of  the 
White  Mountains,  on  the  slopes  of  Mt.  Willard  and  not  far  from 
the  Crawford  House,  the  granite  has  penetrated  an  argillitic  mica 
schist  or  micaceous  slate,  and  the  zones  are  somewhat  differ- 
ent. G.  W.  Hawes  in  1881  established  the  following  seven: 
i.  The  argillitic  mica  schist  (chloritic) ;  2.  Mica  schist  (biotitic) ; 
3.  Tourmaline  hornstone ;  4.  Tourmaline  veinstone  (a  small  con- 
tact band,  rich  in  tourmaline) ;  5.  Mixed  schists  and  granite ; 
6.  Granite  porphyry  (biotitic)  ;  7.  Granite  (hornblendic).  This  is 
one  of  the  most  complete  and  best-exposed  contacts  known,  and 
illustrates  both  external  and  internal  effects.*  The  succession 
illustrates  the  alteration  of  chlorite  to  biotite  by  the  granite,  and 
then  near  the  contact  the  development  of  tourmaline  from  the 
boracic  and  fluoric  emanations  which  were  afforded  by  it.  On 
the  southeast  corner  of  Conanicut  Island,  in  Narragansett  Bay, 
granite  has  penetrated  Carboniferous  shales,  as  described  by  L. 
V.  Pirsson,f  and  has  baked  them  to  compact  hornfels  near 
the  contact.  Spotted  slates  are  likewise  met  resembling  those  de- 
scribed above.  Immediately  beneath  the  diabase  of  the  Palisade 
ridge  at  Hoboken,  N.  J.,  the  Triassic  shales  are  baked  to  a  compact 
hornfels  with  abundant  tourmalines  and  near  Beemerville,  N.  J.,J 

*  Hawes'  paper  is  in  the  American  Journal  of  Science,  January,  1881,  p.  21. 
•j-L.  V.  Pirsson,  On  the  Geology  and  Petrography  of  Conanicut  Island,  R.  I. 
American  Journal  of  Science,  Nov.,  1893,  p.  363. 

J  J.  F.  Kemp,  Trans.  New  York  Acad.  Sci.,  XL,  p.  60. 


128  A   HAND  BOOK  OF  ROCKS. 

a  great  dike  of  nepheline-syenite  has  come  up  through  Ordo- 
vician  shales  and  has  altered  them  in  places  to  remarkably  dense, 
black  hornfels.  Near  Crugers,  on  the  Hudson  River,  mica-diorites 
have  penetrated  mica  schists  and  have  developed  in  them  a  con- 
siderable number  of  characteristic,  contact  minerals,  but  the  changes 
in  the  schists  are  not  specially  apparent  to  the  eye.*  As  western 
and  other  eastern  areas  are  further  studied,  no  doubt  additional 
cases  will  be  fully  described.  Many  are  known  and  await  careful 
field  work. 

The  contact  effects  on  limestones  furnish  extremely  interesting 
phenomena,  involving  a  series  of  minerals  somewhat  different  from 
those  just  described.  Pure  limestones  often  recrystallize  into 
marbles;  but  where  the  contact  minerals  contain  greatly  increased 
percentages  of  silica  and  iron  oxides,  the  introduction  of  these  sub- 
stances from  the  intrusive  mass  is  proven.  The  limestones  then 
become  thickly  charged  with  biotite,  garnet,  vesuvianite,  scapolite, 
pyroxenes  and  amphiboles,  tourmaline,  spinel,  and  not  a  few  more. 
Garnet  and  vesuvianite  are  especially  characteristic.  Good  con- 
tacts have  been  met  at  several  American  localities.  Near  St. 
John,  N.  B.,  f  granite  has  penetrated  Laurentian  limestone  and  has 
developed  a  garnet  zone,  with  more  or  less  pyroxene.  Diorites 
cutting  or  including  limestone  in  the  Cortlandt  series  |  have  caused 
the  formation  of  pyroxene,  scapolite,  hornblende  and  other  minerals. 

In  the  valley  extending  from  Warwick,  N.  Y.,  southwest  to 
Sparta,  N.  J.,  are  most  instructive  exhibitions,  and  rich  mineral 
localities  are  based  on  them.  Granite  is  the  principal  intrusive.  § 
The  western  Adirondack  region  of  New  York  contains  many  more 
where  intrusives  and  limestone  come  together.  The  southwest  and 
Mexico  are  prolific  in  garnet  zones,  which  at  times  contain  cop- 
per mines.  They  have  thrown  much  light  on  ore-deposition. 
Abroad,  the  region  about  Christiania  in  Norway  has  proved  to  be 
classic  ground  for  these  phenomena,  and  a  great  contact  of  diorite 
on  Triassic  limestone  at  Predazzo  in  the  Tyrolese  Alps  has  pro- 
duced the  characteristic  zones  on  a  grand  scale. 

*G.  H.  Williams,  Amer.  Jour.  Set.,  Oct.,  1888,  265. 

fW.  D.  Matthew,  Trans.  N.   Y.  Acad.  Sci.,  XIII.,  194. 

JG.  H.  Williams,  Amer.  Jour.  Sci.,  Oct.,  1888,  267. 

§  J.  F.  Kemp  and  Arthur  Hollick,  Annals  N.   Y.  Acad.  Sci.t  VII.,  644. 


THE  METAMORPHIC  ROCKS.  129 

Increasing  experience,  in  the  West  and  in  Mexico,  has  shown 
that  copper  ores  are  often  deposited  along  the  contacts  of  eruptives 
and  limestone.  Thus  in  the  Seven  Devils  district,  in  western 
Idaho,  bornite  occurs  between  diorite  and  white  marble,  and  is 
mixed  with  epidote  and  garnet  as  a  gangue,  both  being  minerals 
characteristically  developed  in  these  surroundings. 

The  inclusions  of  wall  rock  caught  up  by  an  advancing  intru- 
sion on  its  way  to  the  surface  are  instructive  examples,  and  often 
are  afterwards  found  entombed  in  the  igneous  rock  and  more  or 
less  altered.  The  lava  flows  of  Vesuvius  and  the  ejected  bombs 
have  been  of  extraordinary  interest  in  this  respect.  Limestones 
are  frequent  among  them  and  they  exhibit  the  same  zones  as  the 
larger  occurrences.  Vesuvianite,  in  fact,  received  its  name  from 
this  association. 

Of  the  remaining  members  of  the  grand  division  of  the  Aqueous 
rocks,  the  Carbonaceous  are  the  principal  ones  deserving  mention. 
Coal  seams  of  the  normal  bituminous  variety  have  been  cut  in  not 
a  few  places  by  igneous  dikes,  and  display  in  a  marked  degree  the 
metamorphosing  effect.  The  volatile  hydrocarbons  have  been 
driven  off  and  the  coal  has  become  an  impure  coke.  The  Triassic 
coal  basins  of  Virginia  and  North  Carolina  exhibit  many  instances 
where  diabase  dikes  have  wrought  the  change,  and  in  the  region  of 
Puget  Sound  basalt  intrusions  have  effected  similar  results.  In 
Colorado  and  New  Mexico,  the  near  approach  of  an  igneous  sheet 
has  brought  about  the  formation  of  anthracite,  and  in  fact  all 
grades  of  coal  can  be  detected  from  rich  bituminous  to  hard  an- 
thracite, according  to  the  nearness  of  the  dike  or  laccolite. 

Reference  may  also  be  made  to  the  hills  of  soft  magnetite,  near 
Cornwall,  Pa.,  where  a  great  dike  of  diabase  has  apparently  caused 
the  replacement  of  calcareous  shales  with  pyritous  magnetite. 

Where  intrusions  cut  other  igneous  or  metamorphic  rocks  the 
effects  are  much  less  apparent,  because  the  walls  are  resistant  to 
change,  being  themselves  already  crystalline.  Around  granites, 
however,  even  in  these  conditions,  great  pegmatite  dikes  and  veins 
are  copiously  produced,  which  seems  to  be  in  large  part  brought 
about  by  escaping  heated  vapors  and  solutions. 

Remarkable  cases  of  contact  metamorphism  are,  however,  cer- 
tainly caused  by  these  last  named  agents.  As  rocks  they  are  not 
9 


130  A  HAND  BOOK  OF  ROCKS. 

specially  abundant,  although  of  great  scientific  interest.  Some  in- 
trusions have  emitted  cppious  emanations  of  hydrofluoric  and  bora- 
cic  acid  in  conjunction  with  superheated  steam.  These  vigorous 
reagents  have  attacked  the  wall  rocks,  when  originally  formed  of 
crystalline  silicates,  making  them  porous  and  cellular  from  the  de- 
struction of  feldspars,  and  have  often  caused  the  crystallization  of 
quartz,  tourmaline,  topaz,  fluoric  micas,  fluorite,  apatite  and  other 
characteristic  minerals  of  which  cassiterite  is  of  much  economic 
importance.  Such  metamorphic  products  when  essentially  consist- 
ing of  quartz  and  mica  are  called  greisen.  Tourmaline  granites 
likewise  result.  It  is  not  to  be  overlooked,  however,  that  mineral- 
izers  have  also  played  a  large  part  in  the  cases  earlier  cited,  nor 
should  the  remark  be  omitted  in  conclusion  that  they  and  similar 
agents  have  been  of  very  great  importance  in  the  formation  of 
ores. 


CHAPTER   X. 

THE  METAMORPHIC  ROCKS,  CONTINUED.     THE  ROCKS  PRODUCED 

BY  REGIONAL  METAMORPHISM.     INTRODUCTION.     THE 

GNEISSES  AND  CRYSTALLINE  SCHISTS. 

INTRODUCTION. 

This  subdivision  embraces  rocks  which  differ  widely  among  them- 
selves, but  which  have  nevertheless  important  features  in  common. 
The  following  generalities  are  applicable  in  a  large  way  and  will 
serve  to  emphasize  some  of  the  most  important  points. 

1.  Regionally  metamorphosed  rocks  are  all  more  or  less  per- 
fectly crystalline.     This  is  least  developed  in  the  slates. 

2.  They  are  all  more  or  less  decidedly  laminated  or  foliated, 
although  some  amphibolites,   marbles  and  serpentines  are  quite 
massive.     The  laminations  are  due  to  the  arrangement  of  the  con- 
stituent minerals,  and  especially  the  dark -colored  ones,  in  parallel 
alignment,  so  that  light  and  dark  layers  stand  out  conspicuously. 
The  terms  bedded  and  stratified  should  never  be  applied  to  them 
because  the  banding  is  largely  due  to  dynamical  processes,  and 
has  no  necessary  connection  with  original  sedimentation. 

3.  They  are  of  ancient  geological  age  or  else  are  in  greatly  dis- 
turbed districts. 

It  is  important  in  connection  with  these  rocks  to  distinguish  be- 
tween the  effects  produced  by  heat  or  thermal  metamorphism  and 
the  effects  produced  by  mechanical  forces  or  dynamic  metamor- 
phism. By  thermal  metamorphism  we  understand  the  alterations 
caused  by  heat  not  necessarily  accompanied  by  the  mechanical 
effects  such  as  shearing,  crushing  and  the  like,  that  are  compre- 
hended under  dynamic  metamorphism.  Contact  metamorphism 
is  of  course  a  variety  of  the  former  which,  however,  is  also 
brought  into  play  alike  when  rocks  are  so  deeply  buried  that  they 
come  within  the  sphere  of  influence  of  the  earth's  interior  heat,  and 
when  from  dynamic  stresses,  they  are  crushed  so  that  their  particles 
move  or  slide  under  great  pressure  on  one  another  and  develop 
heat  by  friction.  If  we  imagine  for  a  moment  great  bodies  of 

131 


I32  A   HAND  BOOK  OF  ROCKS. 

rocks  which  have  definite  crushing  resistances,  buried  under  a  load 
of  overlying  strata,  so  deep  within  the  earth  that  their  limits  of  re- 
sistance are  exceeded,  yet  so  confined  that  they  cannot  fly  apart, 
we  perceive  that  they  must  yield  by  internal  crushing,  and  if  the 
upheaval  of  a  mountain  range  eases  the  strain,  that  they  must  flow 
as  a  mass.  It  is  to  this  flow,  accompanied  by  shearing,  that  the 
lamination  of  metamorphic  rocks  is  largely  due.  Prominent  or 
conspicuous  minerals  are  strung  out  in  parallel  lines,  often- 
times with  wavy  folds  and  curves,  and  in  the  end  a  foliated  or 
laminated  structure  is  superinduced  that  suggested  the  bedding  of 
sediments  to  the  early  geologists.  It  is  not  to  be  denied,  however, 
that  the  laminations  do  at  times  correspond  to  original  bedding, 
because  where  the  contrasts  in  chemical  and  mineralogical  compo- 
sition among  the  layers  are  pronounced,  they  doubtless  mark  such 
correspondence,  but  cases  are  well  known  of  old  conglomerate 
beds  passing  directly  across  the  prevailing  schistosity  of  a  gneissic 
district. 

During  these  shearing  and  flow  movements  large  crystals,  such 
as  the  feldspars  of  porphyries,  and  the  larger  uncrushed  nuclei  of 
minerals  in  a  general  pulp  are  squeezed  and  stretched  into  lenses, 
and  remain  like  eyes  between  eyebrows,  so  that  they  are  called 
*'  Augen  "  from  the  German  word  for  eyes.  Swirling  curves  and 
•eddies  in  the  laminations  are  also  familiar  phenomena  and  cannot 
.be  explained  in  any  other  way. 

These  changes  may  take  place  without  mineralogical  alteration, 
as  when  granitoid  rocks  pass  into  gneisses  which  contain  simply  the 
crushed  fragments  of  the  originals,  but  as  a  general  thing  new  com- 
binations are  formed  in  the  metamorphosed  rock.  Pyroxene  passes 
into  hornblende ;  soda-lime  feldspars  become  scapolite  or  saussurite, 
and  other  changes  ensue  which  are  best  detected  with  the  microscope. 
Sedimentary  rocks  suffer  entire  recrystallization,  and  sometimes  so 
thoroughly  lose  their  original  characters  that  no  clue  is  afforded  as 
to  their  history.  In  regional  metamorphism  precisely  as  in  the 
case  of  the  contact  metamorphic  rocks,  it  is  generally  believed  that 
there  is  no  change  in  composition,  except  perhaps  by  the  loss  of 
volatilizable  ingredients,  but  only  rearrangement  of  elements.  A 
gross  analysis  of  a  reasonably  large  sample  will  therefore  give  a 
clue  to  the  composition  of  the  original.  Heated  waters,  generally 


THE  METAMORPHIC  ROCKS.  133 

charged  with  mineral  matter  and  steam,  have  no  doubt  contributed 
largely  in  bringing  about  the  final  results. 

The  Regionally  Metamorphosed  rocks  will  be  described  under 
the  following  heads : 

1.  The  Gneisses  and  Crystalline  Schists. 

2.  The  Quartzites  and  Slates. 

3.  The  Crystalline  Limestones  and  Dolomites:  The  Ophical- 
cites,  Serpentines  and  Soapstones. 

THE  GNEISSES. 

Introductory. — Gneiss  is  an  old  word  which  originated  among  the 
early  German  miners  in  the  Saxon  districts.  It  was  especially  ap- 
plied by  them  to  laminated  rocks  of  the  mineralogical  composition 
of  granite,  and  in  this  sense  it  is  quite  widely  employed  to-day. 
But  there  are  many  important  gneisses  which  correspond  in  min- 
eralogy to  the  other  plutonic  rocks,  and  which  are  quite  as  properly 
designated  by  this  name,  so  that  gneiss  has  come  to  be  a  term  that 
is  of  loose  geological  significance  very  much  as  is  trap,  but  that 
is  none  the  less  useful  for  this  reason.  We  may  therefore  define 
gneiss  as  a  laminated,  metamorphic  rock  which  usually  corresponds 
in  mineralogy  to  some  one  of  the  plutonic  types.  Gneisses  differ 
from  schists  in  the  coarseness  of  the  laminations,  but  as  these 
become  finer  they  pass  into  schists  by  insensible  gradations. 
Varieties  are  sometimes  indicated  by  prefixing  the  name  of  the 
most  prominent  silicate,  usually  one  of  the  ferro-magnesian  group, 
thus  hornblende-gneiss,  biotite-gneiss,  pyroxene-gneiss,  but  we 
also  often  speak  of  quartz-gneiss,  orthoclase-gneiss,  plagioclase- 
gneiss,  garnet-gneiss  and  the  like. 

It  is  evident  at  once  that  the  above  names  are  incomplete. 
Hornblende-gneiss,  for  instance,  does  not  indicate  whether  the 
rock  contains  orthoclase  or  plagioclase,  quartz  or  no  quartz,  and 
the  other  ones  cited  are  open  to  the  same  or  similar  objections,  and 
if  in  the  endeavor  to  embody  fuller  descriptions  we  string  together 
the  names  of  all  the  minerals  in  the  rock,  we  employ  an  objection- 
able and  awkward  method  of  coining  words.  A  system  has,  how- 
ever, been  suggested  by  C.  H.  Gordon,*  in  a  recent  paper  that 
obviates  many  of  these  objections  and  that  is  adopted  below  with 

*  Bulletin  of  the  Geological  Society  of  America,  VII.,  122. 


134 


A   HAND  BOOK  OF  ROCKS. 


some  abbreviation  to  make  it  suitable  for  an  elementary  book.  It 
is  based  on  the  parallelism  which  exists  between  the  mineralogy  of 
gneisses  and  that  of  the  massive  plutonic  rocks,  and  it  avails  itself 
of  the  short  names  of  the  latter,  which  indicate  in  each  case,  a 
definite  combination  of  minerals,  to  describe  the  aggregates  present 
in  the  former.  Two  sedimentary  terms  are  also  added. 


MassireType. 

Gneiss  of  Correspond- 
ing Mineralogy. 

Sedimentary 
Type. 

Derived  Gneiss. 

Granite 
Syenite 
Diorite 
Gabbro 
Pyroxenite 
Peridotite 

Granitic  Gneiss 
Syenitic  Gneiss 
Dioritic  Gneiss 
Gabbroic  Gneiss 
Pyroxenitic  Gneiss 
Peridotitic  Gneiss 

Conglomerate 
Sandstone 

Conglomerate  Gneiss 
Quartzite  Gneiss 

Dr.  Gordon  also  suggests  that,  when  gneisses  are  evidently 
dynamic  derivatives  from  a  massive  rock,  this  relationship  be  in- 
dicated by  using  the  terms  granite-gneiss,  syenite-gneiss  and  so 
on.  If,  however,  differentiations  in  the  magma  before  crystallizing 
have  given  rise  to  laminations,  he  advocates  that  such  be  distin- 
guished by  the  adjective  gneissoid,  as  gneissoid  gabbros. 

Gneisses  are  occasionally  met  which  do  not  exactly  correspond 
to  any  of  the  above  names.  Chlorite,  for  example,  is  a  not  un- 
common mineral,  and  while  it  is  evidently  an  alteration  product 
from  pyroxene,  hornblende  or  biotite,  the  original  mineral  is  not  at 
once  apparent,  and  some  such  name  as  chlorite-gneiss  must  be 
used.  In  the  same  way  cordierite-gneiss  describes  those  rare 
varieties  containing  cordierite  (iolite  and  dichroite  are  synonyms 
of  cordierite) ;  sillimanite-gneiss,  garnet-gneiss,  epidote-gneiss  and 
others  convey  in  their  names  their  characteristic  features. 


ANALYSES  OF  GNEISSES. 

Chemical  analyses  often  enable  us  to  trace  back  gneisses  to  their 
original  rocks,  whether  igneous  or  sedimentary,  but  it  requires 
careful  study  of  correct  type  analyses  and  some  familiarity  with 
their  ranges  in  composition  to  do  it.  So  far  as  their  number 
admits  the  analyses  quoted  on  earlier  pages  will  be  found  sug- 
gestive : 


THE  METAMORPHIC  ROCKS.  135 

SiO,       A1,O,       Fe2O,  FeO        CaO         MgO       K;O  Na,O        Loss  or  H,O 

1.  76.61     12.45        —  i-33      0-84        ••-        5-42         3-12  0.53 

2.  74.95  9.42  7.47  ...            1.65  0.13  2.02             4.05                 1.02 

3.  73.47  15.07  I.I5  4.48  0.12  0.38             5.59 

4.  71.46  15.06  ...  2.43         1.40  0.42  5.17             3.23                0.83 
5-  69.35  18.83  2.00                   5.94  ...                      3.78 

6.  69.94  14.85  7.62           ...  2.10  0.97  4.33  4.30  0.70 

7.  61.96  19.73        •••  4-6°  0.35  1.81  2.50  0.79  1.82 

8.  57.66  22.83  7-74  1-16  3.56  5.72  0.60  1.50 

9.  57.20  19.57  9.52  0.59  5.73  4.40  0.28  2.13  0.88 
10.  54.89  13.67  1.35  5.63  4.70  8.34  1.95  2.76 

I.  Granite  gneiss,  west  side  of  Black  Hills,  4Oth  Par.  Survey,  I.,  p.  no.  R.  W. 
Woodward,  Anal.  2.  Called  a  dioritic  gneiss  in  reference,  contains  hornblende,  quartz, 
plagioclase,  orthoclase.  Idem,  R.  W.  Bunsen,  Anal.  3.  Conglomerate  gneiss, 
so-called  granite ;  Munson,  Mass.  Quoted  by  G.  P.  Merrill,  Stones  for  Building  and 
Decoration,  p.  418.  4.  Granite  gneiss,  Iron  Mountain,  Wyo.,  R.  W.  Woodward, 
Anal.  See  under  No.  I.  5.  Dark  variety  of  No.  3.  6.  Granite  gneiss,  derived 
from  a  hornblende  granite,  Trembling  Mountain,  Quebec,  Fundamental  Laurentian 
of  Logan.  F.  D.  Adams,  Amer.  Jour.  Sci.,  July,  1895,  p.  67.  W.  C.  Adams,  Anal. 
7.  Quartzitic  gneiss,  with  garnet,  sillimanite,  graphite  and  pyrite  ;  St.  Jean  de  Matha, 
Quebec,  Idem,  N.  N.  Evans,  Anal.  8.  Granite  gneiss,  probably  a  metamorphosed 
clay  or  slate.  Trembling  Lake,  Quebec.  Contains  garnets  and  sillimanite,  F.  D. 
Adams,  Amer.  Jour.  Sci.,  July,  1895,  P-  67,  W.  C.  Adams,  Anal.  9.  Dioritic 
gneiss,  New  York  City,  P.  Schweitzer,  Amer.  Chemist,  VI.,  457,  1876.  10.  Gneiss 
containing  malacolite,  scapolite,  orthoclase,  graphite,  pyrite.  Rawdon,  Quebec.  See 
under  No.  8. 

Comments  on  the  Analyses.  —  Nos.  1 ,  4  and  6  are  clearly  derived 
from  granites,  presumably  by  dynamic  metamorphism.  The 
analyses  correspond  closely  in  their  general  features  with  those 
given  on  p.  33  except  that  the  A12O3  of  No.  I  is  a  trifle  low,  and 
the  Fe2O3  of  No.  6  a  trifle  high.  Nos.  3  and  5  are  now  known 
to  be  metamorphosed  Cambrian  conglomerate,  although  so  thor- 
oughly recrystallized  as  to  be  a  well-known,  commercial  granite. 
The  conglomerate  must  have  come  from  granitic  or  dioritic  orig- 
inal rocks.  Nos.  7  and  8  correspond  to  the  analyses  of  slates  as 
noted  by  F.  D.  Adams  in  the  original  reference  (see  also  under 
slates,  p.  145).  No.  10  as  noted  by  Adams  is  of  doubtful  interpre- 
tation. The  high  alkalies,  lime,  magnesia  and  the  moderate  silica 
suggest  a  basic  syenite  or  trachyte,  but  the  alumina  is  exception- 
ally low  for  these.  It  may  be  a  tuff  or  a  slightly  altered  sediment 
from  these  originals.  No.  2  is  a  very  anomalous  rock,  and  it  is 
difficult  to  refer  it  to  an  original  diorite,  it  is  so  high  in  silica  and 
so  low  in  alumina.  The  iron  is  very  large  for  so  acidic  a  rock. 


136  A  HAND  BOOK  OF  ROCKS. 

No.  9  is  undoubtedly  an  altered  sediment  as  indicated  by  the  local 
geology.  Notwithstanding  the  anomalies  of  composition,  chem- 
ical analyses  supply  one  of  the  surest  clues  to  the  geological  his- 
tory of  gneisses  and  it  is  to  be  hoped  that  they  will  be  multiplied 
in  America.  At  present  but  few  are  available,  far  fewer  than  of 
igneous  rocks. 

Alteration. — The  alteration  of  gneisses  is  similar  in  all  respects  to 
that  of  their  corresponding  massive  types.  The  feldspars  alter  to 
kaolin,  the  micas  and  hornblende  to  chlorite  and  the  rock  softens 
down  to  loose  aggregates  that  contribute  heavily  to  the  sedimen- 
tary rocks. 

Distribution. — Gneisses  are  abundant  in  ancient,  geological  for- 
mations. The  early  Archean  is  their  especial  home,  and  they  form 
the  largest  part  of  its  vast  areas  in  Canada,  around  the  Great  Lakes, 
along  the  Appalachians  and  in  the  Cordilleran  region.  But  no 
single  division  of  geological  time  monopolizes  them  any  more  than 
such  an  one  does  plutonic  rocks.  There  are  Cambrian  and  even 
Carboniferous  gneisses  in  New  England,  and  dynamic  metamor- 
phism  may  produce  them  from  massive  rocks  of  almost  any  age. 
The  later  geological  formations  are,  however,  seldom  buried  suf- 
ficiently deep  to  be  in  favorable  situations.  Much  the  same  holds 
true  of  Europe  and  the  rest  of  the  world.  The  gray  and  red 
gneisses  of  the  mining  districts  about  Freiberg,  in  Saxony,  those 
of  the  Highlands  of  Scotland,  those  in  Scandinavia,  and  the  won- 
derful exhibitions  of  dynamic  metamorphism  in  the  Alps  are  to 
be  cited  as  of  unusual  historic  and  scientific  interest. 

Granulite.  —  Granulite  is  a  word  that  has  possessed  somewhat 
contrasted  meanings  according  as  it  has  been  used  in  Germany, 
France  or  England.  In  Germany  as  first  employed  it  was  applied 
to  a  finely  gneissoid  rock  that  consists  chiefly  of  feldspar,  quartz  and 
garnets.  These  original  granulites  have  other  minerals  more  or 
less  prominently  developed,  of  which  cyanite,  augite,  biotiteand  horn- 
blende are  chief.  The  texture  of  the  rock  is  extremely  dense,  and 
except  for  the  garnets,  cyanite  or  augite,  the  individual  minerals  are 
hardly  discernible.  Among  French  and  English  speaking  peoples 
the  name  granulite  has  been  applied  to  granitic  rocks  that  appear  to 
the  eye  to  be  chiefly  quartz  and  feldspar,  although  the  microscope 
may  show  muscovite.  They  are  practically  binary  granites,  or 


THE  METAMORPHIC  ROCKS. 


'37 


rich  quartz  and  feldspar  gneisses.  The  name  has  also  been  used 
for  coarse  plutonic  rocks  that  have  been  crushed  down  by  dyna- 
mic metamorphism  into  a  finely  granular  and  homogeneous  aggre- 
gate. But  so  far  as  metamorphic  rocks  have  been  met  in  America, 
cases  are  very  rare  which  cannot  be  satisfactorily  described  without 
the  use  of  this  word,  which  has  been  so  perverted  from  its  original 
application  as  to  be  practically  valueless  without  an  accompanying 
explanation. 

THE  CRYSTALLINE  SCHISTS. 

The  crystalline  schists  have  finer  laminations  than  the  gneisses, 
but  in  other  respects  the  mineralogy  is  often  much  the  same,  and 
as  already  stated  no  very  sharp  line  can  be  drawn  between  them. 
It  is  important  to  note  that  the  words  "  schiste  "  of  the  French  and 
"  Schiefer  "  of  the  Germans  are  applied  to  shales,  slates  and  meta- 
morphic schists  indiscriminately,  but  in  English  schist  is  only  used 
for  metamorphic  rocks.  The  more  important  schists  are  broadly 
classified,  according  to  the  principal  ferro-magnesian  silicate  that  is 
present,  into  the  following  three  groups  under  which  they  will  be 
taken  up. 

(a)  Mica-schists. 

(£)  Hornblende-schists  or  Amphibolites. 

(r)  Various  Minor  Schists. 

THE  MICA-SCHISTS. 


SiO, 

A1,0S 

Fe,0, 

FeO 

CaO 

MgO 

K,0 

Na,0 

HaO 

I. 

82.38 

11.84 

... 

2.28 

I.OO 

0.83 

0.38 

0.77 

2. 

79.50 

13.36 

2.87 

... 

0.71          < 

3-95 

4.69 

0.36 

0.78 

3- 

69-4S 

14.24 

6.54 

2.66 

•35 

2.52 

4.02 

0.52 

4- 

66.21 

18.60 

5-34 

0.44 

.24 

3-80 

2.16 

2.04 

5. 

62.98 

16.88 

2.48 

5-oo 

tr. 

-58 

7-45 

3-02 

... 

6. 

6i.57 

19-53 

5-44 

2.61 

tr. 

.90 

2.14 

3.48 

... 

7- 

60.49 

»9-35 

0.48 

5.98 

1.08 

.89 

3-44 

2.55 

3-66 

8. 

57-67 

17.92 

9.00 

3-19     : 

(-29 

3-86 

1.09 

3.19 

9- 

55-12 

24.32 

6.73 

4-99 

tr. 

tr. 

2.83 

2.71 

... 

10. 

49.00 

23-65 

8.07 

... 

0.63        c 

>-94 

9.11 

i-75 

3-41 

I.  Mica-schist,  rich  in  quartz,  Monte  Rosa,  Switzerland,  Zulkowsky,  Sitz.  Wiener 
Akad.,  XXXIV.,  41,  1895.  2.  Mica-schist,  with  quartz  and  green  mica,  Zermatt, 
Switzerland.  Bunsen  in  Roth's  Tabellen,  1862.  3.  Garnetiferous  mica-schist  with 
feldspar,  Brixen.  Tyrol.  Schonfeld  and  Roscoe,  Ann.  der.  Chem.  u.  Phar.,  XCI., 
1854,  305.  4.  Mica-schist,  near  Meissen,  Saxony,  Hilger  quoted  in  Roth's  Tabellen, 
1879.  5-  Mica-schist,  Crugers,  N.  Y.,  contains  quartz,  orthoclase,  biotite,  muscovite, 
a  little  oligoclase.etc.  F.  L.  Nason  for  G.  H.  Williams,  Amer.  Jour.  Sci.,  Oct.,  1888. 


1 38  A   HAND  BOOK  OF  ROCKS. 

259.  6.  Crumpled  garnetiferous  mica-schists,  Idem.  7.  Argillitic  mica-schist,  G.  W. 
Hawes,  Geology  of  New  Hampshire,  Part  III.,  219.  8.  Mica-schist  near  Messina, 
Sicily,  Ricciardi,  quoted  in  Roth's  Tabellen,  1884,  p.  ix.  9.  Staurolite  mica-schist, 
with  biotite,  muscovite,  quartz,  sillimanite,  garnet.  See  under  No.  6.  10.  Sericite 
schist,  Wisconsin,  Wis.  Geol.  Surv.,  I.,  304. 

Comments  on  the  Analyses.  —  Like  the  majority  of  gneisses  the 
mica-schists  are  more  or  less  closely  parallel  with  the  granites  in 
chemical  composition  because  the  constituent  minerals  are  so 
largely  the  same  in  both.  But  where  they  have  been  formed  from 
metamorphosed  sediments  such  as  shales,  clays,  and  the  like,  the 
alkalies  are  often  lower  than  in  the  case  of  siliceous  igneous  rocks, 
and,  what  is  still  more  characteristic  of  sediments  as  contrasted 
with  highly  siliceous  igneous  rocks,  the  magnesia  is  in  excess  of 
the  lime.  A  comparison  of  the  above  analyses  with  those  of  the 
rhyolites,  trachytes,  granites  and  syenites  earlier  given  will  forcibly 
bring  this  out.  The  local  geology  as  well  as  the  analyses,  indicate 
that  there  is  little  doubt  that  Nos.  5,  6,  7  and  9  are  altered  sedi- 
ments, and  the  presumption  is  strong  that  almost  all  the  others 
are  also. 

Mineralogical  Composition,  Varieties.  — The  most  prominent  and 
abundant  minerals  in  the  mica-schists  are  quartz,  muscovite  and 
biotite.  While  they  are  more  or  less  interleaved  together,  yet 
close  examination  of  the  coarser  varieties  shows  that  they  are  in 
layers  irregularly  parallel  and  to  a  large  extent  distinct.  The 
minerals  are  in  all  degrees  of  relative  abundance,  quartz  sometimes 
largely  predominating  and  marking  a  passage  to  the  quartzites, 
while  again  the  micas  may  be  in  great  excess.  Both  muscovite 
and  biotite  are  met,  the  former  being,  perhaps,  rather  the  more 
abundant.  With  these  chief  minerals  are  almost  always  associated 
very  considerable  amounts  of  feldspar,  both  orthoclase  and  plagio- 
clase,  and  variable  proportions  of  garnet,  staurolite,  cyanite,  silli- 
manite, tourmaline,  apatite,  pyrite  and  magnetite. 

The  garnet  and  staurolite  may  exhibit  surprisingly  well  devel- 
oped crystals  and  illustrate  the  extraordinary  power  of  certain 
compounds  to  crystallize  under  circumstances  apparently  ill-adapted 
to  their  perfect  development. 

Mica-schists  embrace  a  series  from  rather  coarsely  crystalline 
varieties  to  others  that  are  excessively  fine-grained  and  that  are 
near  relatives  of  the  slates.  The  minerals  of  the  latter  may  be  of 


THE  METAMORPHIC  ROCKS.  139 

microscopic  dimensions,  and  only  the  aggregate  of  shining  scales 
reveals  them  as  mica.  Such  aggregates,  of  a  silvery  white  color 
but  of  composition  essentially  the  same  as  normal  muscovite,  are 
called  sericite,  and  the  corresponding  schists,  sericite-schists.  A 
soda-mica  (muscovite  and  its  relatives  are  potash  micas)  is  called 
paragonite.  Hydromica  is  a  name  applied  many  years  ago  by  Dana 
to  sericite,  paragonite,  and  perhaps  others  resembling  them,  so 
that  for  these  finely  micaceous  schists,  especially  in  our  eastern 
states,  hydromica  schist  is  a  field  name  that  has  been  largely  used 
in  practice  and  in  geological  reports.  These  fine-grained  mica- 
schists  that  approximate  slates  are  also  made  a  special  group  by 
many,  under  the  name  phyllite,  a  very  useful  term  and  one  to  be 
strongly  commended.  Mica-schists  are  also  met  that  are  high  in 
lime  and  that  mark  transitions  to  the  crystalline  limestones.  The 
abundance  of  calcite  or  dolomite  betrays  them,  and  to  such  the 
names  calcareous  schist  or  calc-schist  are  applied. 

Mica-schists  result  from  the  thorough  metamorphism  or  recrys- 
tallization  of  sandstones,  shales  and  clays,  and  also  from  the  crush- 
ing and  excessive  shearing  of  igneous  rocks,  granitoid  and  por- 
phyritic  alike.  A  possible  origin  from  ancient  volcanic  tuffs  is 
always  to  be  considered  in  the  study  of  a  district,  but  the  questions 
of  origin  are  obscure  and  are  subjects  for  thorough  chemical  and 
microscopical  investigation. 

Alteration. — The  mica-schists  are  rather  resistant  to  alteration 
and  often  appear  on  mountain  tops.  When  alteration  does  prevail, 
they  soften  to  masses  of  quartz  sand,  chlorite  scales  and  kaolin. 

Distribution.  —  The  mica-schists  form  the  country  rock  over  vast 
areas  in  New  England  and  to  the  south  along  the  eastern  Appa- 
lachians. Although  long  regarded  as  of  uncertain  or  obscure  geo- 
logicaj  relations  they  are  now  recognized  as  being  in  large  part  at 
least  metamorphosed  Cambrian  and  Ordovician  shales  or  related 
sediments.  Around  Lake  Superior  and  in  the  regionally  meta- 
morphosed areas  of  the  West  they  are  not  lacking. 

THE  HORNBLENDE  SCHISTS  OR  AMPHIBOLITES. 

Introductory.  —  Under  dynamic  metamorphism  the  basic  igneous 
rocks  whose  chief  bisilicate  is  pyroxene,  pass  very  readily  into 
hornblendic  rocks,  with  a  greater  or  less  development  of  schistosity. 


140  A   HAND  BOOK  OF  ROCKS. 

On  account  of  the  prevailing  parallel  arrangement  of  the  prismatic 
crystals  of  hornblende,  schistosity  is  seldom  entirely  lacking,  but 
where  less  distinct  the  name  amphibolite  has  proved  to  be  a  useful 
alternative,  and  indeed  is  of  wide  general  application.  Sedimentary 
rocks  are  also  known  in  rarer  instances  to  yield  similar  results. 


SiO, 

A12O, 

Fe3Os 

FeO 

CaO 

MgO 

K,O 

Na,O 

H20 

I. 

52-39 

16.13 

1.64 

1.44 

8.76 

4.70 

1.42 

2.59 

0.17 

2. 

50-44 

8.18 

1.  06 

6.28 

"•55 

17.63 

0.50 

2.98 

0.98 

3- 

49.19 

I8.7I 

5-°3 

4.04 

5-92 

7.98 

0.77 

1-44 

5-05 

4- 

46.31 

11.14 

21.69 

9.68 

tr. 

6.91 

4-44 

5- 

44-49 

16.37 

5-07 

5-50 

7-94 

7-50 

0.56 

2-59 

4-99 

I.  Hornblende-schist,  Grand  Rapids,  Wis.,  Geology  of  Wis.,  IV.,  629.  Also,  Fe  in 
pyrite,  0.34  ;  S,  0.39  ;  P,O5,  0.28  ;  Ca  in  apatite  0.815.  2-  Pseudo-diorite  of  Becker, 
Knoxville,  Calif.,  Monograph  XIII.,  U.  S.  Geol.  Surv.,  101,  W.  H.  Melville,  Anal. 
Also,  MnO  0.213,  Cr2Oj  0.480.  3.  Hornblende-schist  derived  from  gabbro,  Lower 
Quinnesec  Basin,  Wis.  R.  B.  Riggs  for  G.  H.  Williams,  Bull.  62,  U.  S.  Geol.  Surv., 
p.  89.  Also,  CO,  1.82.  4.  Hornblende-schist  near  Cleveland  Mine,  Mich.,  Foster 
and  Whitney,  Rept.  on  the  Iron  Lands  of  Lake  Superior,  p.  92.  5.  Hornblende- 
schist,  Lower  Quinnesec  Falls,  Wis.,  R.  B.  Riggs  for  G.  H.  Williams,  Bull.  62,  U.  S. 
Geol.  Surv.,  p.  91.  Also  CO3  5.38. 

Comments  on  the  Analyses.  —  The  analyses  indicate  basic  rocks, 
of  decidedly  variable  composition.  Nos.  3  and  5  are  certainly 
sheared  igneous  rocks.  No.  2  is  regarded  by  Becker  as  a  meta- 
morphosed sediment.  It  is  quite  different  from  the  others  in  its  low 
alumina,  and  its  great  excess  of  magnesia  over  lime.  No.  I  appears 
to  be  an  altered  igneous  rock  and  No.  4  is  probably  the  same. 
Aside  from  exhibiting  the  composition  of  these  rocks,  the  analyses 
are  interesting  when  compared  with  those  of  the  basic  diorites  (p. 
64)  and  the  gabbros  and  pyroxenites  (pp.  78,  8l). 

Mineralogical  Composition,  Varieties.  —  The  most  abundant  min- 
eral in  these  rocks  is  naturally  hornblende.  With  it  are  associated 
oftentimes  biotite,  augite,  plagioclase,  garnet,  magnetite,  pyrite  and 
pyrrhotite ;  but  quartz,  except  as  forming  veinlets,  is  not  often  met 
nor  is  it  to  be  expected  in  such  basic  rocks.  The  commonest  va- 
riety of  hornblende  is  black  to  the  eye,  but  is  green  in  thin  section. 
It  forms  prismatic  crystals  from  moderately  coarse  to  microscopi- 
cally fine.  The  prisms  are  interlaced  so  as  to  make  a  very  tough 
aggregate  and  one  that  breaks  with  difficulty  under  the  hammer. 
Light  green  actinolite  may  also  form  schists.  Black  scales  of  bio- 
tite appear  interlaminated  with  the  hornblende.  The  augite  is  not 


THE  METAMORPHIC  ROCKS.  141 

readily  distinguished  from  the  hornblende  with  the  eye  alone.  It 
is  in  large  degree  the  remnants  of  original  pyroxenes  that  have 
partially  passed  into  hornblende  during  the  metamorphic  process. 
The  plagioclase  also  represents  to  a  great  extent  the  feldspar  that 
was  in  the  original  gabbro  or  other  igneous  rock  from  which  the 
amphibolite  has  been  derived.  The  plagioclase  is  often  replaced 
by  secondary  products,  such  as  epidote,  calcite,  scapolite  and 
others,  which  together  make  up  the  aggregate  formerly  called 
saussurite,  and  regarded  as  an  individual  mineral.  The  minor 
accessories,  magnetite,  pyrite,  pyrrhotite  and  garnet  deserve  no 
special  mention.  Except  magnetite,  which  never  fails,  they  are  of 
more  or  less  irregular  occurrence. 

Alteration. — The  hornblende  passes  readily  into  chlorite  and 
softens  to  a  scaly  mass  with  the  separation  of  much  limonite  that 
yields  a  characteristic,  rusty  outcrop.  If  any  pyrite  or  pyrrhotite 
is  present  it  greatly  expedites  the  alteration  by  its  contribution  of 
sulphuric  acid.  The  feldspars  yield  calcite  and  kaolin  and  the 
whole  mass  becomes  a  rusty  clay  or  soil. 

Occurrence.  —  The  hornblende-schists  constitute  individual  belts 
in  schistose  regions  in  the  midst  of  other  metamorphic  rocks  and 
also  great  areas  by  themselves.  Dikes  and  sheets  of  diabase  and 
plutonic  masses  of  gabbro  in  districts  that  have  been  subjected  to 
violent  dynamic  upheavals  readily  pass  into  them.  The  same 
areas  in  the  Eastern  States  that  were  cited  under  gabbro  contain 
them,  and  they  are  minor  members  in  the  schistose  districts  of  New 
England.  Around  Lake  Superior  they  form  a  most  important  part 
of  the  geology  of  the  iron  ore  regions,  and  in  the  Black  Hills,  the 
Rocky  Mountains  and  the  ranges  of  California  they  are  often  met. 

VARIOUS  MINOR  SCHISTS. 

Under  this  collective  term  are  assembled  a  series  of  minor  rocks, 
no  one  of  which  compares  in  importance  with  the  schists  already 
mentioned,  but  all  of  which  may  be  met  as  subordinate  members  of 
metamorphic  districts.  There  are  also  others  in  considerable  variety 
which  are  esteemed  too  unimportant  for  an  elementary  book. 

SiO,  Al,0,  Fe.O,  FeO       CaO  MgO  K,O         Na.O         H,O 
Chlorite  schist. 

1.  49.18  15-09  12.90  10.59  5.22  1.51         3.64        1.87 

2.  47.10  2.14  44.33  0.36  0.13  5.19 


I42  A   HAND  BOOK  OF  ROCKS. 

Jhi«.        SiO,  Al.O,  Fe,0s  FeO          CaO           MgO  K,O          N«tO         H,O 

3.             58.66  9.26  4.42  0.94        22.78  4.09 

4-             5°-81  4-53  3-52  4-26                        31.55  4.42 
Epidote  schist. 

5.  41.28  18.48  9.44  8.20        7.04        7.48  2.21         3.52         2.74 
Eclogite. 

6.  48.89  14.46  2.00  7.15        13.76     12.21  0.17         1.75        0.40 
Glaucophane  schist. 

7.  47.84  16.88  4.99  5.56        11.15       7-89  0.46        3.20         1.98 

I.  Chlorite  schist,  Klippe,  Sweden,  Cronqvist  for  Tornebohn.  Quoted  by  Roth, 
Gesteinsanalysen,  1884,  p.  viii.  2.  Chlorite  schist,  Foster  Mine,  Mich.,  C.  F. 
Chandler,  Geol.  of  Mich.,  I.,  91.  3.  Talc  schist,  Fahlun,  Sweden,  Uhde  quoted  by 
Roth,  Gesteinsanalysen,  1861,  56.  4.  Talc  schist,  Gastein,  Austria,  R.  Richter. 
Idem.  5.  Epidote  schist  from  diabase,  South  Mountain,  Pa.,  C.  H.  Henderson, 
Trans.  Amer.  Inst.  Min.  Eng.,  XII.,  82.  6.  Eclogite,  Altenburg,  Austria,  Schuster, 
Tscher.  Mitt.,  1878,  368.  7.  Glaucophane  schist,  Monte  Diablo,  Calif.,  W.  H.  Mel- 
ville, Bull.  Geol.  Soc.  Amer.,  II.,  413. 

Comments  on  the  Analyses.  —  These  analyses  are  too  variable  to 
admit  of  much  in  the  way  of  comparative  remarks,  for  the  rocks 
are  so  totally  unlike.  No.  I  suggests  an  original  diabase  or  some 
such  rock.  No.  2  is  abnormally  rich  in  iron,  doubtless  in  large 
part  from  magnetite  or  hematite.  The  high  magnesia  in  Nos.  3 
and  4  is  characteristic  and  indicates  their  close  relations  with  ser- 
pentines. No.  5  is  an  altered  diabase.  No.  6  is  of  a  rock  variable 
in  its  mineralogy  and  obscure  in  its  history.  No.  7  is  practically 
a  hornblende-schist  with  glaucophane,  an  amphibole  that  is  high  in 
soda,  instead  of  common  hornblende. 

Mineralogical  Composition,  Varieties.  —  The  chlorite  schists  are 
marked  by  the  presence  of  this  green  micaceous  mineral  in  large 
amount.  More  or  less  quartz  is  also  generally  present,  and  not  in- 
frequently plagioclase,  talc,  epidote  and  magnetite.  The  schistose 
texture  is  pronounced.  The  chlorite-schists  are  manifestly  altera- 
tion products  from  some  rock,  with  abundant,  anhydrous,  iron- 
alumina  silicates.  Hornblende-schists,  presumably  from  basic 
igneous  rocks  are  the  general  source.  Certain  chlorite-schists  are 
often  called  "  green  schists." 

Talc-schists  are  characterized  by  sufficient  talc  to  make  this 
mineral  prominent  and  in  addition  they  have  quartz  as  the  next 
most  abundant  constituent.  Feldspar  may  at  times  be  noted,  and 
some  micaceous  mineral  is  not  rare.  Care  is  necessary  not  to  con- 
fuse fine  scales  of  the  last  named  with  talc  itself.  Various  accessory 


THE  METAMORPHIC  ROCKS.  143 

minerals  likewise  occur,  and  the  magnesian  carbonates,  dolomite 
and  magnesite  are  often  present.  Obviously  the  talc-schists  have 
resulted  from  the  alteration  of  some  rock  with  one  or  more  anhy- 
drous, magnesian  silicates  that  lacked  iron.  Tremolite  and  ensta- 
tite  are  the  most  available,  but  the  original  sources  of  these  are 
often  obscure.  Siliceous  dolomites  or  intrusive  pyroxenites  at 
once  suggest  themselves,  but  the  iron  must  of  necessity  have  been 
low,  so  as  not  to  yield  serpentines. 

Epidote-schists  result  when  the  ferro-magnesian  silicates  and  the 
plagioclases  are  so  favorably  situated  with  reference  to  each  other 
as  to  establish  mutual  reactions.  They  especially  arise  as  phases 
in  the  metamorphism  of  pyroxenic  or  hornblendic  rocks,  such  as 
diabase,  hornblende-schists  and  the  like.  Eclogite  is  a  rock  scarcely 
known  in  America,  having  as  yet  only  been  noted  in  the  Mar- 
quette  District,  Mich.  (Geol.  of  Wis.,  III.,  649)  and  in  California. 
It  is  a  well  recognized  variety,  however,  in  Europe.  It  consists  of 
bright  green  amphiboles  and  pyroxene,  of  garnet  and  of  a  variety 
of  minor  minerals.  In  ordinary  determination  it  would  not  be 
distinguished  from  a  garnetiferous,  actinolite  schist.  Glaucophane 
is  a  blue  soda  amphibole  that  is  rare  in  America,  except  in  the 
Coast  range  of  California,  where  it  characterizes  certain  important 
schists.  The  rocks  have  a  pronounced  blue  shade,  and  contain  in 
addition  quartz  and  feldspar.  In  California  they  certainly  are  al- 
tered shales.  Graphite  appears  quite  commonly  as  a  characteristic 
mineral  of  certain  schists,  and  may  justify  the  use  of  the  name 
graphite  schist.  More  or  less  mica,  and  always  quartz  and  feld- 
spar are  associated. 

Distribution.  —  Chlorite-schist  and  talc-schist  are  not  uncommon 
members  of  our  larger  metamorphic  series,  especially  along  the 
Appalachians  in  New  England  and  around  Lake  Superior. 
Epidote-schist  is  less  common  in  the  same  relations.  The  occur- 
rence of  eclogite  and  glaucophane-schist  has  already  been  cited. 
Graphite-schist  is  not  infrequent  in  the  metamorphosed  Paleozoic 
strata  of  the  East. 


CHAPTER  XI. 

THE  METAMORPHIC  ROCKS,  CONTINUED.     THE  ROCKS  PRODUCED 
BY  REGIONAL  METAMORPHISM.     THE  QUARTZITES  AND 
SLATES.     THE  CRYSTALLINE  LIMESTONES  AND 
DOLOMITES,  OPHICALCITES,  SERPEN- 
TINES  AND   SOAPSTONES. 

THE  QUARTZITES. 

SiO,  Al,0,    Fe.O,      FeO  CaO         MgO  KaO         Na,O         H,O      Sp.  Gr. 

1.  97.1          1.39     1.25  0.18        0.13 

2.  96.44  1.74  0.33  0.17  0.22  0.13  0.19  0.90 

3.  84.52       12.33  2-12        o-S1          tr-          °-XI        °-34        2-31        2-74 
I.  Quartzite,  Chickies  Station,  Penn.,  Penn.  Geol.   Surv.  Rep.  M.,  p.    91.      2. 

Sandstone  partly  altered  to  Quartzite,  Quarry  Mtn.,  Ark.,  R.  N.  Bracket!  for  L.  S. 
Griswold,  Geol.  of  Ark.,  1890,  III.,  140,  161.  3.  Quartzite,  Pipestone,  Minn.,  W. 
A.  Noyes  in  Minn.  Geol.  Surv.,  I.,  198. 

Comments  on  the  Analyses. — There  is  no  essential  difference  in 
the  analyses  of  quartzites  and  sandstones,  as  the  few  quoted  above 
will  show,  but  doubtless  the  resulting  quartzite  is  somewhat  richer 
in  silica  than  the  original  sandstone.  Comparatively  few  analyses 
of  quartzites  have  been  made  in  America. 

Mineralogical  Composition,  Varieties. — The  quartzites  are  meta- 
morphosed sandstones,  and  differ  from  the  latter  principally  in  their 
greater  hardness,  and  to  a  certain  extent  in  their  fairly  pronounced 
crystalline  character.  These  qualities  are  due  to  the  presence  of 
an  abundant  siliceous  cement  that  is  crystalline  quartz,  and  that  is 
often  deposited  around  the  grains  of  quartz  of  the  original  sand- 
stone, so  as  to  continue  their  physical  and  optical  properties.  The 
original  grains  have,  therefore,  had  the  power  of  controlling  the 
orientation  of  the  molecules  of  the  new  silica  as  it  crystallized. 
When  the  original  sandstone  has  been  argillaceous  the  resulting 
quartzite  contains  mica  and  especially  muscovite,  and  with  increase 
of  the  mica,  such  quartzites  pass  through  the  intermediate  varieties 
of  quartz-schist  into  mica-schists.  A  very  curious  and  more  or 
less  micaceous  variety  is  the  so-called  flexible  sandstone  or  itacol- 
umite,  whose  grains  have  the  power  of  slight  movement  on  one 
another  from  their  loosely  interlocked  arrangement,  so  that  thin 

144 


THE  METAMORPHIC  ROCKS.  145 

slabs  may  be  bent  through  a  considerable  arc.  Quartzites  also 
result  from  pebbly  sandstones  and  conglomerates,  and  the  pebbles 
of  these  latter  are  often  flattened  by  the  dynamic  movements  with 
which  the  metamorphism  is  at  times  associated.  There  is  no  sharp 
line  of  demarcation  between  quartzites  and  sandstones,  and  while 
the  extremes  of  soft  sandstones  and  hard  quartzites  are  entirely  dif- 
ferent, the  determination  of  intermediate  varieties  is  more  or  less 
arbitrary. 

Alteration.  —  Quartzites  sometimes  soften  to  sand  on  their  out- 
crops, and  in  the  process,  almost  the  last  vestiges  of  alumina  or 
lime  may  be  removed.  In  this  way  the  sands  in  analysis  No.  I, 
p.  97,  were  formed.  In  general,  however,  they  are  excessively 
resistant  rocks,  and  tend  to  form  prominent  ledges. 

Distribution.  —  Quartzites  occur  in  almost  all  series  of  metamor- 
phosed sediments,  and  as  these  are  best  developed  in  the  later 
Archean  (Huronian,  Algonkian)  strata,  they  especially  characterize 
them.  In  the  metamorphic  belt  in  New  England  and  along  the 
Appalachians,  they  are  frequent,  as  well  as  in  the  Huronian, 
around  Lake  Superior  and  Lake  Huron  and  in  the  similar  areas 
of  the  West. 

THE  SLATES. 


SiO, 

AlaOs 

Fe,0, 

,           FeO 

CaO 

MgO 

KaO 

NaaO 

HtO 

I. 

66.45 

13.38 

I.7I 

1.41 

2.86 

6.28 

0.05 

0.90 

4-03 

2. 

66.00 

24.60 

tr. 

tr. 

3-67 

2.22 

3-00 

3- 

65-85 

16.65 

5-31 

0.59 

2.95 

3-74 

1-31 

3.10 

4- 

64.57 

I7.30 

7.46 

1.16 

2.60 

1.99 

... 

4.62 

5- 

63-31 

16.16 

3-79 

0.15 

4-44 

7.56 

1.54 

.2.65 

6. 

60.50 

19.70 

7.83 

1.  12 

2.20 

3-18 

2.20 

3-3° 

7- 

60.32 

23.10 

7-05 

... 

0.87 

3-83 

0.49 

4.08 

8. 

57.00 

20.  10 

10.98 

1.23 

3-39 

i-73 

1.30 

4.40 

9- 

55.88 

21.85 

9-03 

0.16 

1.49 

3-64 

0.46 

3-39 

10. 

54.80 

23-15 

9.58 

1.06 

2.16 

3-37 

2.22 

3-90 

I.  Slate,  Llanberis,  Wales.  Quoted  by  G.  P.  Merrill,  Stones  for  Building  and 
Decoration,  p.  421,  also  MnO,  0.91,  COV  1.30.  2.  Slate,  Etchemin  Riv.,  N.  B.,  T. 
S.  Hunt,  Phil.  Mag.  (4),  VII.,  237,  1854.  3.  Roofing  slate,  Westbury,  Can.,  Idem. 
4.  Roofing  slate,  Lehesten,  Germany.  Frick,  quoted  by  Roth,  Gesteinsanalysen,  1861, 
P-  57-  5-  Damourite  slate,  Hensingerville,  Pa.,  Geol.  of  Penn.,  Rep.  M.,  91.  6. 
Roofing  slate,  Wales,  T.  S.  Hunt  as  under  No.  2.  7.  Slate,  Lancaster  Co.,  Penn., 
also  FeSj,  0.09.  See  under  No.  i.  8.  Roofing  slate,  Angers,  France,  T.  S.  Hunt,  as 
under  No.  2.  9.  Blue  black  carbonaceous  slate,  Peach  Bottom  slate,  York  Co.,  Penn., 
also  MnO  0.586,  CoO  tr.,  C  1.974,  FeS,  0.51,  SOS  0.022.  See  under  No.  I.  IO. 
Roofing  slate,  Kingsey,  Quebec,  T.  S.  Hunt,  as  under  No.  2. 
10 


146  A  HAND  BOOK  OF  ROCKS. 

Comments  on  the  Analyses. — The  analyses  are  especially  signifi- 
cant when  compared  with  those  of  the  shales  and  clays,  p.  100, 
and  with  those  of  the  mica-schists,  p.  137,  with  which  latter  they 
are  closely  parallel.  Two  features  at  once  impress  the  observer, 
the  excess  of  magnesia  over  lime,  and  the  excess  of  potash  over 
soda.  The  former  stamps  their  origin  as  from  sediments  rather 
than  from  igneous  rocks  of  these  percentages  in  silica,  because  this 
relative  excess  of  magnesia  as  noted  under  the  mica-schists  is 
rather  characteristic  of  sediments. 

Miner alogiccd  Composition,  Varieties. — As  the  sandstones  during 
metamorphism  pass  into  quartzites,  so  the  shales  and  clays  become 
slates,  when  not  so  thoroughly  recrystallized  as  to  yield  mica- 
schists  or  phyllites.  The  more  sandy  shales  afford  varieties  that 
break  irregularly  and  that  lack  homogeneity,  but  tough  and  even 
slates  result  from  homogeneous  clays  and  are  among  the  most 
remarkable  of  rocks.  The  distinctive  feature  of  slates  as  against 
shales  is  the  possession  of  a  new  cleavage  that  may  lie  at  any 
angle  with  the  original  bedding  of  the  rock,  and  that  has  no  defi- 
nite relation  to  it.  The  cleavage  has  been  developed  by  dynamic 
strains  that  have,  beyond  question,  involved  a  shearing  stress  and 
some  differential  movement  among  the  layers,  though  it  may  have 
been  microscopic.  As  a  matter  of  observation  the  component 
grains  of  slates  have  become  flattened  and  lie  parallel  with  the  new 
cleavage,  and  any  mica  flakes  or  hornblende  needles  that  may  be 
present  lie  along  it. 

Various  explanations  have  been  advanced  for  slaty  cleavage,  and 
its  artificial  production  in  different  substances  has  occupied  several 
investigators.  Based  principally  upon  experiments  performed  by 
Professor  John  Tyndall,  over  forty  years  ago,  it  has  been  usually 
referred  to  a  compressive  force  at  right  angles  to  its  plane.  Tyndall 
subjected  blocks  of  wax  to  pressure,  using  wet  glass  plates  as  his 
buttress  of  resistance.  The  blocks  were  of  course  greatly  reduced 
in  thickness  and  were  forced  to  spread  or  bulge  laterally.  Shortly 
afterward  H.  C.  Sorby,  partly  on  the  basis  of  the  flattening  of  the 
component  grains,  and  the  alignment  of  mica  scales,  explained  the 
cleavage  as  due  to  planes  of  weakness  caused  by  this  new  arrange- 
ment. Recently,  G.  F.  Becker  of  the  U.  S.  Geological  Survey  has 
repeated  the  experiments  of  Tyndall  with  modifications.  So  long 


THE  METAMORPHIC  ROCKS.  147 

as  the  resisting  glass  plates  were  wet  with  water  the  slaty  cleavage 
was  developed,  but  when  they  were  smeared  with  a  heavy  lubricat- 
ing oil,  although  there  was  lateral  expansion  during  compression, 
no  bulging  took  place  and  no  cleavage  was  developed.  Manifestly 
therefore  the  frictional  drag  of  the  plates  enters  into  the  problem, 
and  although  the  resolution  of  the  forces  involved  is  somewhat 
complex,  a  shearing  stress  results  that  is  a  strong  factor  in  pro- 
ducing the  cleavage.*  In  the  case  of  the  large  beds  or  strata 
which  are  metamorphosed  into  slate  in  Nature,  the  case  is  even 
less  simple,  and  the  contrasts  in  rigidity,  between  the  beds  that 
yield  slates,  and  their  enclosing  strata,  are  less  pronounced  than  in 
the  experiment,  but  there  is  little  doubt  that  the  compression  and 
slight  lateral  flow  which  occasion  a  flattening  of  the  grains  and  an 
alignment  of  the  scaly  minerals  across  the  direction  of  application 
of  the  force  in  this  way  produce  the  cleavage.  All  slates  have 
cross-cleavages,  or,  it  may  be,  joints,  more  or  less  well  developed, 
and  one  of  these  may  even  be  perfect  enough  in  connection  with 
the  regular  cleavage,  to  cause  the  slate  to  break  into  small  prisms 
available  for  slate  pencils,  for  which  in  earlier  years  they  were  em- 
ployed. All  slate  quarries  also  show  curly  slates,  where  quartz- 
veins  or  sandy  and  harder  streaks  in  the  original  sediment  have 
caused  imperfections  in  the  cleavage.  It  has  been  noted  that  in 
some  quarries  the  available  plates  appear  to  become  thicker  in 
depth,  as  if  the  surface  weathering  had  been  a  factor  in  developing 
the  cleavages.  Though  commonly  drab  to  black,  they  may  be 
red,  green  or  purple. 

Slates  pass  by  all  intermediate  gradations  into  phyllites  and 
mica-schists.  The  word  slate  is  also  loosely  used  for  shales  that 
have  never  had  any  secondary  cleavage  induced  in  them,  and  this 
is  especially  true  of  the  black,  bituminous  shales  that  occur  with 
coal  seams,  but  in  strict,  geological  use,  the  new  cleavage  and 
metamorphism  should  be  essentials  of  a  true  slate. 

Alteration.  —  Slates  are  exceedingly  resistant  as  is  shown  by 
their  use  in  thin  slabs  for  roofs,  and  they  often  constitute  prominent 
ledges  or  even  peaks.  They  soften  down  to  a  clay  in  the  last  stages 
of  alteration,  but  always  on  the  outcrop  are  more  tender  than  in 

*  G.  F.  Becker,  Finite  Homogeneous  Strain,  Flow  and  Rupture  in  Rocks,  Bull.  Geol. 
Soc.  Amer.,  IV.,  82,  1893. 


148  A   HAND  BOOK  OF  ROCKS. 

depth,  so  that  much  dead  work  is  unavoidable  m  opening  quarries. 
Distribution.  —  Our  most  prominent  slates  are  Cambrian  or  Or- 
dovican  in  age.  Along  the  Green  Mountains  and  especially  in 
northern  Vermont  they  are  strongly  developed.  Again  in  eastern 
Pennsylvania,  in  Virginia  and  in  Georgia  they  are  met  in  great 
areas.  On  the  south  shore  of  Lake  Superior  merchantable  grades 
have  been  somewhat  developed.  Along  the  western  slopes  of  the 
Sierra  Nevada  Mountains  they  are  very  important  rocks. 

THE  CRYSTALLINE  LIMESTONES  AND  DOLOMITES. 

Loss 


C«CO,  MgCO,  SiO,  Al.O,  FO  Insol.  HO 

1.  99'5I  O.29  O.2O 

2.  99-24  0.28  '  -  .  -  • 

3-  98.43  o-3°  o-3»  0-38  0.15 

4-  98-21  2.35  0.15  0.35 

5-  98.00  0.57  1.63 

6.  96.82  1.89  o.  10  2.12 

7.  92.42         6.47  0.35  0.95 

8.  70.1  25.40  2.40 

9.  54.62         45.04  o.io  0.7 

10.          54.25          44.45  0.60 

I.  Statuary  Marble,  Brandon,  Vt.  Quoted  by  G.  P.  Merrill,  Stones  for  Building  and 
Decoration,  417.  2.  Marble,  Carrara,  Italy,  Idem.  3.  Marble,  Knoxville,  Tenn., 
Idem,  also  S,  0.014,  Organic  Matter,  0.068.  4.  Cross-grained  black  and  white  mot- 
tled Marble,  Pickens  Co.,  Ga.,  locally  called  Creole  ;  Geol.  Surv.  Ga.,  Bulletin  I.,  87. 
5.  White  Marble,  Rutland,  Vt.,  see  under  No.  i.  6.  Coarsely  crystalline  white  Mar- 
ble, Cherokee  Quarry,  Pickens  Co.,  Ga.,  see  under  No.  4.  7.  White  Crystalline 
Limestone,  Franklin  Furnace,  N.  J.,  Geo.  C.  Stone,  unpublished.  8.  Crystalline 
Magnesian  Limestone,  Tuckahoe,  N.  Y.,  H.  L.,  Bowker  for  Lime  Co.  9.  Crystalline 
Dolomite,  so-called  "Snowflake  Marble,"  Pleasantville,  N.  Y.,  i6th  Ann.  Rep.  Dir. 
U.  S.  Geol.  Survey,  Part  IV.,  p.  468.  10.  Crystalline  Dolomite,  white  marble,  Inyo 
Co.,  Calif.,  Ann.  Rep.  Calif.  State  Mineralogist,  i<28. 

Comments  on  the  Analyses.  —  The  analyses  do  not  differ  essen- 
tially from  those  of  unaltered  limestones  except  in  so  far  as  the  ones 
in  the  table  are  purer  carbonates  of  lime  and  magnesia.  The  avail- 
able analyses  are  of  merchantable  marbles,  and  in  the  nature  of  the 
case  these  are  derived  from  very  pure  sedimentary  limestones. 
They  are  interesting  as  illustrating  a  series  from  a  rock  that  is 
almost  chemically  pure  carbonate  of  lime  to  one  in  which  the 
carbonate  of  magnesia  reaches  the  values  of  typical  dolomite. 
Comparison  with  the  analyses  of  limestones  earlier  given,  on  p.  1051 
is  recommended.  It  will  be  seen  that  in  this  case  there  is  apparently 


THE  METAMORPHIC  ROCKS.  149 

no  change  in  gross  composition  because  of  metamorphism,  but  of 
course  the  relations  of  the  silica  and  the  bases  are  different.  In  the 
sedimentary  limestones  the  silica  is  largely  in  the  form  of  quartz  and 
in  combination  with  alumina  forming  hydrated  silicates,  such  as 
kaolin.  In  the  crystalline  limestones  it  is  largely  in  silicates  of 
lime,  magnesia  and  alumina,  such  as  tremolite,  pyroxene,  phlogo- 
pite,  etc.,  minerals  whose  formation  has  been  one  of  the  results  of 
metamorphism.  The  percentages  in  the  insoluble  column  do  not 
therefore  indicate  pure  silica.  There  may  be  even  microscopic, 
barite  crystals  present. 

Mineralogical  Composition,  Varieties. — The  crystalline  limestones 
and  dolomites  are  metamorphosed  forms  of  the  sedimentary  varie- 
ties earlier  described.  The  change  involved  is,  as  the  name  im- 
plies, one  of  crystallization.  Fossils,  and  to  a  large  degree  bedding 
planes,  are  destroyed  and  a  more  massive  aggregate  of  calcite  or 
dolomite  crystals  results.  Such  carbonaceous  material  as  was 
originally  present  usually  affords  streaks  of  graphite  which  occasion 
dark  veinings.  They  bring  out  the  brecciation  or  flow-lines  in- 
duced by  the  pressure  from  the  mountain  making  upheavals 
usually  attendant  on  the  metamorphism.  Other  bituminous  or 
ferruginous  matter  may  yield  pronounced  colors  of  many  hues. 

If  the  original  limestone  has  been  an  impure  variety  and  has 
contained  silica,  alumina  and  iron  oxides,  as  illustrated  by  the 
analyses  on  p.  105,  these  components  have  furnished  the  neces- 
sary materials  for  the  various  silicates  that  the  metamorphism 
has  caused  to  form.  Tremolite  is  a  common  result,  light-colored 
pyroxenes  are  not  infrequent,  and  phlogopite  and  other  micaceous 
minerals  are  the  most  abundant  of  all.  Large  quarries  always 
show  borders  or  streaks  that  are  characterized  by  these  minerals, 
and  where  the  original  limestone  passed  into  shales  or  sandstones 
at  its  upper  and  under  surfaces,  these  micaceous  varieties  are 
almost  always  met.  For  ornamental  purposes,  the  included  sili- 
cates serve  to  mar  the  stone,  being,  except  in  the  case  of  micas,  of 
greater  hardness  than  the  calcite. 

Crystalline  limestones  form  more  or  less  extensive  strata  in  the 
midst  of  other  metamorphic  rocks.  Slates,  phyllites,  mica-schists 
and  quartzites  are  their  most  common  associates.  The  dolomites 
may  have  formed  in  many  cases  from  pure  calcareous  limestones  by 


ijo  A   HAND  BOOK  OF  ROCKS. 

the  infiltration  of  magnesian  solutions,  and  by  an  exchange  of  a  por- 
tion of  the  magnesia  for  a  portion  of  the  lime,  as  earlier  referred  to 
on  page  106,  but  so  many  unaltered  limestones  are  high  in  mag- 
nesia, that  the  change  is  not  a  necessary  attendant  of  metamorphism. 

Alteration. — Crystalline  limestones  are  soluble  rocks  and  weather 
with  comparative  facility.  Where  they  occur  in  metamorphic 
belts  they  are  invariably  in  the  valleys,  and  are  potent  factors  in 
determining  the  direction  of  the  drainage  lines.  Where  exposed 
for  long  periods  they  afford  a  coarse,  crumbling  sand  or  gravel, 
that  is  much  used  for  roads  in  the  borders  of  the  Adirondacks  and 
in  western  New  England.  The  final  stage  is  a  mantle  of  residual 
clay  from  which  the  calcareous  material  has  been  largely  leached. 

Occurrence.  —  The  crystalline  limestones  are  frequent  in  our 
metamorphic  districts.  In  the  Appalachian  belt  they  are  of  great 
areal  and  economic  importance,  and  are  largely  quarried  in  Ver- 
mont, Massachusetts,  New  York,  Pennsylvania  and  Georgia.  In 
western  Colorado  they  are  strongly  developed,  and  in  the  Sierras 
of  California  the  same  is  true,  Inyo  County  being  a  rather  large 
producer  of  marble.  The  foreign  mountainous  and  metamorphic 
districts  exhibit  enormous  exposures.  The  great  series  of  ranges 
which  begin  in  the  Pyrenees  and  extend  through  the  Alps  and  the 
Carpathians  to  the  Himalayas,  have  many  famous  quarries  and 
ledges.  The  region  of  the  "  Dolomites  "  in  the  Tyrolese  Alps  is 
a  district  of  especial  richness.  The  Carrara  marble  of  the  Ap- 
penines,  the  Pentelic  of  Greece  and  the  colored  varieties  from 
Northern  Africa,  indicate  their  presence  in  those  regions. 

THE  OPHICALCITES,  SERPENTINES  AND  SOAPSTONES. 


Ophicalc.  CaCO, 

MgCO, 

CO,' 

SiO, 

MgO 

H20 

FeO 

Al.O, 

I. 

57-37 

9.64 

0.74 

13.18 

10.29 

4.06 

3-57 

0.85 

2. 

23-85 

22.28 

1.97 

22.42 

18.74 

6-43 

4-30 

3- 

7-65 

10.98 

1.78 

36.53 

28.08 

8.63 

6.49 

Serp. 

SiO, 

MgO 

H,0 

Al.O, 

Cr,0,        Fe.O, 

FeO 

NiO 

CaO 

4- 

44.14 

42.97 

12.89 

5- 

43.87 

38.62 

9-55 

0.31 

7-17 

0.27 

O.O2 

6. 

42.52 

42.16 

14.22 

I. 

96 

7- 

41.54 

40.42 

14.17 

2.48 

i-37 

0.04 

8. 

40.67 

32.61 

12.77 

5-»3 

8.12 

9- 

40.06 

39.62 

12.  10 

i-37 

0.20 

3-43 

0.71 

10. 

36.95 

33-07 

10.40 

16. 

So 

n. 

34.84 

30-74 

17-39 

0.42 

0.68           6.08 

1.85 

tr. 

7.02 

THE  METAMORPHIC  ROCKS.  151 

Soapst.  SiO,  MgO  H,0  Al.O,         Cr.O.        Fe.O,         FeO          NiO         MnO 

12.  64.44  33.19  0.34  0.48  1.39         0.33 

13.  62.10  32.40  2.05  1.30  2.15 

14.  62.00  33.1  4.9 

i.  Ophicalcite,  Oxford,  Quebec,  T.  S.  Hunt,  Amer.  Jour.  Sci.,  March,  1858,  220. 
The  analysis  as  cited  is  assembled  from  several  partial  analyses.  2.  Ophicalcite, 
Brompton  Lake,  Quebec,  Idem,  p.  221.  Original  results  recast  as  in  No.  I.  3.  Ophi' 
calcite,  Brompton  Lake,  Quebec,  Idem,  p.  222.  Recast  as  before.  4.  Theoretic^ 
serpentine,  HiMgsSi,O,.  5.  Massive  serpentine,  Webster,  N.  C.,  F.  A.  Genth,  Amer 
Jour.  Sci.,  II.,  xxxiii.,  201.  6.  Massive  serpentine,  Montville,  N.  J.,  E.  A.  Manice, 
Dana's  Mineralogy,  1877,  467.  7.  Serpentine,  a  metamorphosed  sandstone,  New 
Idria,  Calif.,  W.  H.  Melville  for  G.  H.  Becker,  in  Monograph  XIII.,  U.  S.  Geol. 
Surv.,  1 10.  8.  Serpentine,  decomposed  peridotite,  Syracuse,  N.  Y.,  T.  S.  Hunt, 
Amer.  Jour.  Sci.,  Sept.,  1858,  237.  9.  Serpentine,  Dublin,  Harford  Co.,  Md.  Quoted 
by  G.  P.  Merrill,  Stones  for  Building  and  Decoration,  414.  10.  Serpentine  from  peri- 
dotite, Presq'  Isle,  Mich.,  J.  D.  Whitney,  Amer.  Jour.  Sci.,  II.,  xxviii.,  18,  also  Na,O, 
0.97.  II.  Serpentine  from  peridotite,  Monte  Diablo,  Calif.,  W  H.  Melville,  Bull, 
Geol.  Soc.  Amer.,  II.,  408,  also  Nap,  0.42,  K,O,  0.07.  12.  Soapstone,  Webster, 
Jackson  Co.,  N.  C.,  F.  A.  Genth,  Minerals  of  North  Carolina,  p.  6l.  13.  Talc,  Gouver- 
neur,  N.  Y.,  Analysis  quoted  by  C.  H.  Smyth,  Jr.,  School  of  Mines  Quarterly,  July, 
1896,  p.  340.  14.  Theoretical  talc,  6MgO,  5SiO2,  2H2O. 

Comments  on  the  Analyses.  —  The  ophicalcites  mark  a  passage 
from  the  dolomites  to  the  serpentines.  They  are  practically  crys- 
talline magnesian  limestones  or  dolomites,  which  are  mottled  with 
inclusions  of  serpentine  in  varying  amounts.  The  analyses  begin 
with  one  that  is  over  half  calcite  and  over  two  thirds  calcite  and 
dolomite.  The  ratios  of  the  remaining  oxides  are  just  about  those 
required  by  serpentine.  In  the  second  the  amount  of  serpentine 
has  much  increased,  and  in  the  third  the  carbonates  have  notably 
retreated.  Under  the  serpentines,  as  compared  with  the  theoret- 
ical mineral,  No.  4,  the  succeeding  analyses  are  all  notably  rich  in 
iron.  Except  in  the  cases  of  Nos.  10  and  1 1,  they  are  remarkably 
uniform  considering  their  diverse  origin.  In  No.  10  the  SiO2 
drops,  probably  from  the  presence  of  magnetite,  while  in  the  last 
the  pyroxene  of  the  original  peridotite  has  contributed  consider- 
able lime.  In  all  these  rocks  A12O3  is  notably  low.  It  is  most 
abundant  in  No.  8,  a  serpentine  that  is  derived  from  a  rock  with 
much  augite.  Chromium  is  a  rather  characteristic  element  in  ser- 
pentines which  result  from  basic  igneous  rocks,  and  nickel  can  be 
very  generally  detected  on  analysis.  Lime  practically  fails  except 
in  No.  n.  It  should  be  appreciated  that  as  a  mineral,  serpentine 
is  a  unisilicate,  whereas  talc  is  a  bisilicate,  and  this  explains  the 


152  A   HAND  BOOK  OF  ROCKS. 

much  larger  percentage  of  silica  in  the  latter.  The  soapstones  are 
fairly  pure,  aggregates  of  talc,  as  a  comparison  of  Nos.  1 2  and  1 3 
with  No.  14  will  indicate. 

Mineralogical  Composition,  Varieties.  —  The  ophicalcites  are  mot- 
tled rocks  consisting  of  irregular  or  rounded  masses  of  green  ser- 
pentine embedded  in  white  calcite  and  dolomite.  The  proportions 
of  the  constituent  minerals  are  variable.  The  serpentine  may  be  in 
small  nodules  a  fraction  of  an  inch  in  diameter  or  in  large  stringers 
and  masses  several  feet  across.  This  irregularity  renders  it  dif- 
ficult in  quarrying  to  preserve  a  uniform  grade.  The  stone  is 
mottled  green  and  white,  and  when  uniform  is  a  very  beautiful  one. 
The  serpentine  varies  from  dark  green  or  almost  black,  to  light 
clear  shades,  and  has  been  derived  in  a  number  of  cases,  as  has 
been  shown  by  G.  P.  Merrill,*  from  original  pyroxene  crystals. 

The  ophicalcites  are  therefore  in  many  cases  alteration  products 
from  a  crystalline  limestone,  that  has  been  surcharged  with  pyrox- 
enes, and  this  itself  may  probably  be  referred  in  most  cases  to  an 
original  siliceous,  magnesian  sediment,  recrystallized  by  regional 
metamorphism. 

Ophicalcites  are  also  called  ophiolites,  serpentinous  marbles  and 
verd  antique.  The  syllable  "  ophi,"  in  all  these  words  is  derived 
from  the  Greek  for  serpent  and  ophicalcite  means  therefore  a  ser- 
pentinous limestone. 

The  serpentines  are  green  or  red  aggregates  of  scales,  fibers  or 
massive  individuals  of  the  mineral  serpentine.  They  display  con- 
siderable variety  of  texture  according  to  the  characters  of  these 
components.  Other  minerals  are  not  especially  prominent.  Grains 
of  chromite  or  magnetite  may  be  detected  and  garnets  of  the 
variety  pyrope  are  sometimes  well  developed.  Veinlets  of  calcite 
or  of  magnesian  carbonates  ramify  through  the  rock  in  many  expo- 
sures. Remains  of  the  original  olivine,  pyroxene,  or  hornblende 
from  which  the  serpentine  has  been  derived  may  often  be  detected 
and  biotite  or  some  hydrated  magnesian  mica  is  not  infrequent. 
The  varieties  of  the  mineral  serpentine  are  numerous,  but  many  of 
them  are  too  rare  to  be  serious  rock-makers.  Almost  all  serpen- 
tines have  been  formed  by  the  alteration  of  basic  igneous  rocks, 

*  G.  P.  Merrill,  Amer.  Jour.  Sci.,  March,  1889  ;  Proc.  U.  S.  Nafl  Museum,  XII., 
595,  1890. 


THE  METAMORPHIC  ROCKS.  153 

among  which  the  pyroxenites  and  peridotites  are  the  chief  con- 
tributors. Hornblende  schists  also  yield  them  and  G.  F.  Becker 
has  recorded  the  remarkable  case  of  sandstones  that  pass  into  them 
in  the  Coast  Ranges  of  California. 

Soapstones,  called  also  steatites,  are  chiefly  talc  as  the  analyses 
show.  Quartz  veinlets  often  run  through  the  rock  and  scattered 
grains  of  quartz  are  not  infrequent.  Magnesian  carbonates  are 
likewise  evident  in  many  exposures.  In  the  case  of  the  Gouver- 
neur  beds  of  talc  (see  Anal.  13),  C.  H.  Smyth  has  shown  that  the 
original  minerals  have  been  tremolite  and  enstatite,  and  that  the 
beds  occur  in  crystalline  limestone,  but  it  is  a  hard  problem  to  de- 
termine from  what  the  tremolite  and  enstatite  have  been  derived. 
Two  reasonable  sources  suggest  themselves,  either  a  siliceous  dolo- 
mite, or  a  non-ferruginous,  basic  intrusive.  The  soapstones  are 
not  particularly  abundant  rocks  but  are  of  economic  value  where 
met.  They  are  close  relatives  to  the  talc  schists  earlier  cited. 

Alteration. — The  serpentinous  rocks  themselves  are  thoroughly 
altered  derivatives  from  fresher  anhydrous  ones  and  in  their  further 
decomposition  simply  soften  to  incoherent  dirt  and  clay.  The 
more  resistant,  included  minerals  are  thus  set  free,  and  as  in  the 
case  of  platinum  and  garnets  they  may  be  concentrated  in  gravel. 

Distribution.  —  Ophicalcites  are  most  abundant  in  Quebec,  the 
northern  Green  Mountains  and  the  foothills  of  the  Adirondacks. 
The  serpentines  are  especially  notable  on  Staten  Island,  in  south- 
eastern Pennsylvania  and  the  neighboring  parts  of  Maryland,  where 
the  gabbros,  as  stated  on  p.  84,  and  their  related  rocks  are  abun- 
dant. They  share  in  an  important  belt  of  these  basic  intrusives  in 
North  Carolina  and  Georgia.  In  the  basic  igneous  rocks  around 
Lake  Superior  they  are  occasionally  met  as  alteration  products. 
In  the  Coast  ranges  the  serpentines  are  of  very  great  importance, 
and  in  part  are  altered  sediments.  They  are  likewise  common 
abroad,  and  in  a  minor  capacity  appear  in  many  metamorphic  dis- 
tricts. Soapstone  is  much  less  common,  but  is  met  in  this  country 
as  a  subordinate  member  in  much  the  same  regions  as  the  serpen- 
tines and  crystalline  dolomites. 


CHAPTER   XII. 

THE  METAMORPHIC  ROCKS,  CONCLUDED.     THE  ROCKS  PRODUCED 

BY  ATMOSPHERIC  WEATHERING.     THE  DETERMINATION 

OF  THE  METAMORPHIC  ROCKS. 

Introduction.  —  It  is  a  matter  of  common  observation  that  out- 
crops of  rocks  and  loose  boulders  are  always  more  or  less  decom- 
posed and  broken  down  or  "  weathered  "  for  a  greater  or  less  dis- 
tance below  their  surfaces.  This  may  not  be  serious  enough  to 
prevent  the  accurate  recognition  of  the  rock,  and  usually  within 
the  area  once  covered  by  the  great  ice  sheet  of  the  Glacial  Period 
it  is  not,  because  the  moving  ice  has  ploughed  away  all  loose  and 
decomposed  materials,  but  south  of  the  terminal  moraine,  and  above 
all  in  the  tropics,  the  decomposition  is  excessive  and  may  produce 
to  a  depth  of  a  hundred  feet  or  more  a  mass  of  alteration  products 
that  give  of  themselves  slight,  if  any,  clue  to  their  originals.  This 
is  a  common  experience  in  the  Southern  States,  where,  as  well  as 
in  Central  and  South  America,  the  indefinite  character  of  the  surface 
rock  throws  great  difficulties  in  the  way  of  accurate  geological  map- 
ping. So  difficult  at  times  is  the  determination  of  the  country  rock, 
that,  for  example,  during  field  work  in  Brazil,  O.,  A.  Derby  has 
felt  compelled  to  resort  to  the  panning  out  of  the  surface  materials 
with  a  gold-seeker's  pan  in  order,  by  concentrating  the  heavy  but 
small  and  undecomposed,  accessory  minerals,  such  as  zircon,  titanite, 
monazite,  xenotime,  apatite  and  others,  to  get  from  their  character- 
istic associations  some  clue  to  the  original  rock.  Many  travelers 
have  noted  the  brilliant  colors  of  the  soils  of  latitudes  toward  the 
equator  -and  the  comparatively  somber  tones  of  those  toward  the 
poles. 

These  products  of  weathering  are  so  widespread,  therefore,  and 
so  individual  that  a  few  pages  have  been  reserved  for  their  particular 
mention.  Special  names  for  them  have  been  suggested  at  various 
times.  The  oldest  one  and  the  one  most  current  is  laterite.  The 
word  means  brick  earth  and  was  originally  applied  to  the  red  or 
brown  iron-stained  surface  soils  occurring  in  the  tropical  lands,  and 

«S4 


THE  METAMORPHIC  ROCKS.  155 

derived  by  direct  decomposition  from  the  country  rock  in  place. 
It  has  been  applied  in  later  years,  however,  to  all  sorts  of  these  sur- 
face soils  from  whatever  rocks  derived,  and  whether  colored  red  or 
not  G.  F.  Becker,  of  the  U.  S.  Geological  Survey,  has  recently 
(1895)  proposed  saprolite*  a  word  meaning  literally  rotten  rock, 
as  "  a  general  name  for  thoroughly  decomposed,  earthy,  but  un- 
transported  rock."  This  is  practically  the  modern  use  of  laterite, 
although  it  is  broader  than  the  latter's  original  application.  The 
U.  S.  Geological  Survey  in  the  invaluable  series  of  atlas  sheets  now 
being  issued  employs  the  term  surficial,  i.  e.,  surface  rocks,  as  a 
general  designation  for  these  untransported  products  of  decom- 
position. We  also  often  speak  of  residual  clay  as  was  done  on  pp. 
100  and  102  for  the  less  soluble,  aluminous  residues  left  behind  in 
the  removal  of  the  more  soluble  portions  of  limestones. 

The  general  scope  and  application  of  these  names  having  been 
set  forth,  a  brief  consideration  will  be  given  to  the  mineralogical 
processes  of  change  that  have  produced  them  from  several  of  the 
commoner  groups  of  rocks. 

The  chief  causes  of  this  superficial  breaking  down  or  "  degenera- 
tion," as  it  has  been  aptly  called  by  G.  P.  Merrill,f  are,  the  chem- 
ical action  of  rain  and  ground-waters,  especially  when  charged  with 
carbonic  acid  or  other  dissolved  matter ;  organic  life,  both  vege- 
table and  animal,  operating  through  the  agency  of  the  organic 
acids  produced  by  their  living  processes  or  by  their  decomposing 
remains ;  and  the  mechanical  disintegration  produced  by  changes 
of  temperature,  by  the  freezing  of  water  and  by  swelling  from 
hydration  or  from  some  of  the  chemical  or  mineralogical  changes 
among  those  referred  to  above.  Although  having  no  connection 
with  these  atmospheric  processes,  yet  hot  springs  and  allied  exhala- 
tions from  dying  volcanic  energy  bring  about  closely  similar  re- 
sults and  are  able  to  change  great  sheets  of  volcanic  rock  to  bril- 
liantly variegated  masses  of  clay  and  kaolin.  At  the  Falls  of  the 
Yellowstone  River,  in  the  National  Park,  these  are  wonderfully  and 
impressively  displayed,  more  than  a  thousand  feet  of  rhyolite  having 
been  changed  practically  to  kaolin. 

*Gold  Fields  of  the  Southern  Appalachians,  ibtk  Ann.  Rep.  Dir.  U.  S.  Geol. 
Survey,  3,  289,  1895. 

^Bulletin  of  the  Geological  Society  of  America,  VII.,  378. 


156  A  HAND  BOOK  OF  ROCKS. 

Under  the  action  of  the  chemical  agents  the  more  easily  soluble 
elements  are  removed  or  put  in  such  relations  to  one  another  as  to 
facilitate  their  rearrangement  in  new  and  secondary  combinations. 
In  the  rocks  composed  of  silicates  the  most  vulnerable  oxides  are 
lime,  magnesia,  potash  and  soda.  Iron  oxides  also  suffer  ex- 
tensively, but  the  ferric  form  is  sometimes  very  resistant.  Silica 
yields  more  or  less,  especially  to  the  alkaline  solutions  from  the 
potash  and  soda  referred  to  above.  Alumina,  on  the  whole,  is 
least  readily  attacked  of  all,  and  is  usually  the  one  that  furnishes 
the  best  basis  of  comparison  between  analyses  of  altered  and  un- 
altered materials. 

Among  the  igneous  and  metamorphic  rocks,  open  or  porous 
varieties  naturally  suffer  more  than  compact  and  finely  crystalline 
ones.  Rocks  high  in  the  bases  that  are  most  readily  attacked 
chemically,  are  easier  victims  than  those  especially  rich  in  the  re- 
sistant ones.  Basic  rocks,  therefore,  with  their  high  percentages 
of  lime  and  magnesia  and  their  relatively  low  silica,  suffer  espe- 
cially, whereas,  granites  and  related  gneisses  are  much  more  stub- 
born subjects,  the  large  amount  of  quartz  in  them  furnishing  a  very 
resistant  component. 

Granites,  syenites,  acid  diorites  and  their  corresponding  porphy- 
ritic  types  alter  especially  through  the  feldspathic  member  present. 
The  constituent  quartz  is  but  slightly  affected,  and  the  dark  silicates 
are  not  present  in  sufficiently  large  amounts  to  be  very  serious  fac- 
tors. The  resulting  product  is  a  kaolinized  or  clayey  mass  through 
which  are  distributed  quartz  grains,  and  which  is  more  or  less 
stained  by  the  hydrated  oxide  of  iron  that  is  yielded  to  some  ex- 
tent by  the  dark  silicates.  The  characteristic  products  of  the  latter 
are  also  present  in  small  amounts,  but  are  more  extensively  men- 
tioned subsequently.  The  exposed  ledges  furnish  loose  pieces 
that  often  weather  in  concentric  shells  and  simulate  rounded,  water- 
worn  boulders.  The  next  result  is  a  large  contribution  of  clay  and 
sand  to  sedimentary  or  eolian  deposits,  it  may  be  at  a  great  distance. 

In  the  basic  igneous  or  metamorphic  rocks  the  dark  ferro-mag- 
nesian  and  aluminous  silicates  are  in  excess,  and  in  decomposition 
their  peculiar  products  predominate.  The  distinctively  magnesian 
ones  yield  serpentine,  the  aluminous  change  to  chlorite.  Both 
these  minerals  are  prevailingly  green,  and  dark  green,  surficial 


THE  METAMORPHIC  ROCKS.  157 

rocks  result.  The  abundance  of  iron  in  them  leads  to  the  forma- 
tion of  very  rusty  outcrops. 

In  the  case  of  limestone,  the  lime  and  magnesia  are  dissolved 
away,  while  the  alumina,  silica  and  iron  oxides  remain  behind  in 
the  mantle  of  impure  residual  clay  already  referred  to.  The  other 
sedimentary  rocks  suffer  especially  from  mechanical  processes, 
although  chemical  changes  are  not  lacking  among  them,  for,  as  re- 
marked on  page  145  regarding  analysis  No.  I,  of  page  97,  during 
the  breaking  up  considerable  leaching  may  result  that  leads  to  the 
production  of  nearly  chemically  pure  quartz  sand. 

The  mechanical  and  associated,  chemical  breaking  down  of  rocks 
tends  to  place  them  in  more  favorable  conditions  for  further  chem- 
ical alterations,  and  for  erosion  and  removal. 

All  the  changes  in  the  weathering  of  rocks  have  been  well  de- 
scribed by  M.  E.  Wadsworth  as  "  resulting  from  the  general  dissi- 
pation and  degradation  of  the  potential  energy  of  the  constituents 
of  the  earth's  crust  in  the  universal  passage  of  matter  from  an  ac- 
tive state  towards  a  passive  and  inert  condition."  * 

THE  DETERMINATION  OF  THE  METAMORPHIC  ROCKS. 
The  rocks  resulting  from  contact  metamorphism  are  rather  of 
local  interest,  than  of  wide,  areal  distribution.  The  spotted  schists 
and  slates,  and  the  hornstones  are  readily  recognized  by  a  practiced 
observer.  The  crystalline  limestones  even  when  charged  with 
silicates  may  closely  resemble  the  products  of  regional  metamor- 
phism. In  dealing  with  the  latter,  familiarity  with  well  character- 
ized types  is  the  safest  guide.  The  gneisses  are  at  once  apparent 
from  their  laminated  character  and  granitoid  texture.  Transition 
members  between  them  and  the  mica-schists  on  the  one  hand, 
and  the  hornblende-schists  on  the  other,  may  cause  hesitation  as 
to  which  group  they  belong.  The  finely  laminated  ones  are  cer- 
tainly members  of  the  schists,  those  with  prevailing  mica  belong- 
ing with  the  mica-schists,  those  with  prevailing  hornblende,  with 
the  hornblende-schists.  Again  as  the  fineness  of  the  lamina- 
tion or  foliation  increases,  the  schists  pass  into  the  phyllites  and 
slates,  that  are  easily  recognized.  The  quartzites  likewise  present 

*The  Theories  of  Ore  Deposits,  Proc.  Bost.  Soc.  Nat.  Hist.,  Vol.  XXIIL,  p.  202, 
1884. 


1 58  A   HAND  BOOK  OF  ROCKS. 

little  difficulty  as  they  are  practically  hard  sandstone.  The  crys- 
talline limestones  and  dolomites  are  only  to  be  distinguished  by 
the  ease  or  difficulty  of  obtaining  effervescence.  The  ophicalcites 
look  like  no  other  rocks,  and  the  serpentines  and  soapstones  are 
also  at  once  apparent.  The  soapy  feel  of  all  these  magnesian 
rocks  aids  in  their  recognition.  There  are,  of  course,  rare  and  ob- 
scure, metamorphic  rocks  that  cause  trouble,  but,  just  as  in  the 
case  of  the  finely  crystalline  igneous  rocks,  they  are  best  referred 
to  someone  familiar  with  the  use  of  the  microscope. 

THE  STUDY  OF  THE  METAMORPHIC  ROCKS. 

The  metamorphic  rocks  present  some  of  the  most  elusive  and 
difficult  problems  of  geology,  but  in  later  years  the  combination 
of  exact  determinations  of  minerals  by  means  of  the  microscope 
and  of  chemical  analyses  has  been  illuminating.  Strong  efforts 
have  been  made  to  express  by  accurate  chemical  equations  the 
reactions  which  have  taken  place  in  the  re-crystallization  of  min- 
erals, both  at  and  below  the  surface  of  the  earth.  President  C. 
R.  Van  Hise,  of  the  University  of  Wisconsin,  has  brought  together 
a  vast  number  of  these  in  the  Monograph  on  Metamorphism  No. 
XLVII.  of  the  U.  S.  Geological  Survey.  The  book  is  encyclopedic 
as  a  work  of  reference.  With  the  author  we  may  regard  the  outer 
portion  of  the  earth  down  to  the  level  of  standing  water,  as  the 
zone  or  belt  of  weathering  or  oxidation.  Within  it,  rocks  and 
minerals  break  down,  as  just  described  under  the  products  of 
weathering.  The  results  and  processes  of  the  zone  may  be  called 
katamorphic  in  reference  to  the  breaking  down.  Such  of  the  dis- 
solved matter  as  does  not  run  off,  sinks,  and  passing  to  the  deeper 
zone  beneath  the  ground-water  shares  in  the  production  of  new 
minerals  under  the  influence  of  heat  and  pressure.  We  thus  have 
the  deeper  "  building-up  "  or  anamorphic  belt,  called  also  the  zone 
of  cementation.  Fragmental  rocks  are  changed  from  incoherent 
particles  to  solid  masses  by  the  deposition  of  new  material.  These 
terms  have  all  come  into  wide  use  in  later  years  so  that  it  is  well 
for  the  student  to  become  familiar  with  them.  Others  have  further 
sought  to  classify  the  component  minerals  with  regard  to  depth 
of  formation.  Professor  F.  Becke,  now  of  Vienna,  suggested  the 
interesting  plan  of  classifying  the  minerals  into  plus  and  minus 


THE  METAMORPHIC  ROCKS.  159 

groups,  according  as  their  densities  have  increased  or  decreased 
over  the  densities  of  what  might  be  considered  their  components. 
The  increase  or  decrease  is  determined  by  the  aid  of  certain  num- 
bers expressive  of  "  molecular  volumes."  Thus  quartz,  SiO2,  has 
a  molecular  weight  of  60,  and  a  sp.  gr.  of  2.66.  If  we  divide  the 
former  by  the  latter  (60  -4-  2.66)  we  obtain  22.56  expressive  of  its 
molecular  volume.  Corundum,  A12O3  has  a  molecular  weight  of 
102  and  a  sp.  gr.  of  4.  Its  molecular  volume  is  therefore  25.5. 
Now  there  are  three  different  metamorphic  minerals  produced  by 
the  union  of  A12O3  and  SiO2  with  a  molecular  weight  of  162,  viz., 
andalusite,  sp.  gr.  3.20;  molecular  volume,  50.6;  sillimanite,  sp. 
gr.  3.24;  molecular  volume,  50,  and  cyanite,  sp.  gr.  3.6;  mole- 
cular volume  45.  The  sum  of  the  molecular  volumes  of  quartz 
and  corundum  is  48.06,  so  that  if  we  imagine  the  combination  of 
the  two  to  Al2O3,SiO2,  there  is  a  gain  in  molecular  volume, 
respectively  in  andalusite  and  sillimanite  of  2. 54  and  1 .94 ;  whereas 
in  cyanite  there  is  a  loss  of  3.06.  We  would  perhaps  infer  with 
reason  that  cyanite  forms  under  greater  pressure  than  the  other 
two,  and  therefore  at  greater  depth.  By  taking  the  metamorphic 
minerals  comprehensively  and  interpreting  them  in  this  way  with 
regard  to  depth,  some  interesting  inferences  may  be  drawn. 

The  metamorphism  of  ancient  sediments  varies  greatly  in  degree. 
The  history  of  some,  such  as  slates  and  quartzites,  is  not  so  diffi- 
cult to  trace,  but  where  we  meet  others,  very  old,  of  former  deep 
burial,  and  penetrated  through  and  through  with  igneous  rocks, 
the  geologist  finds  it  difficult  to  draw  the  line  between  meta- 
morphosed sediments  and  intrusive  masses.  Recrystallization 
from  the  influence  of  magmatic  waters,  the  so-called  "juice  of  the 
magma ";  "  digestion  or  half-digestion "  by  the  igneous  rock  ; 
thorough  penetration  of  the  sediments  by  igneous  substance, 
yielding  injected  gneisses  ;  and  similar  processes  must  be  invoked. 
The  rocks  of  the  Adirondacks  and  of  Quebec,  and  those  of  Sweden 
and  Finland  have  driven  observers  to  these  conclusions. 

When  dealing  with  metamorphic  districts  the  student  fresh 
from  the  laboratory  must  be  prepared  for  many  obscure  phenomena, 
about  which  the  best  informed  and  most  experienced  workers  are 
not  yet  entirely  clear. 


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CHAPTER   XIII. 
THE   RE-CALCULATION  OF  THE  CHEMICAL  ANALYSES  OF  ROCKS.* 

The  importance  of  chemical  analyses  of  rocks  in  general  and  of 
igneous  varieties  in  particular  has  only  been  properly  appreciated 
within  the  last  ten  years.  It  is  indeed  true  that  in  the  time  of 
Abich  and  Bunsen  in  the  fifth  and  sixth  decades  of  the  last  century 
much  attention  was  given  to  this  branch  of  investigation,  and  that 
the  work  and  influence  of  the  latter  made  available  many  results  ; 
but  interest  languished  with  the  passing  away  of  faith  in  his  two 
fundamental  magmas  —  the  normal -trachytic  and  the  normal- 
pyroxenic  —  in  the  igneous  rocks,  and  the  analyses  which  were 
subsequently  made  and  recorded  were  either  prompted  by  their  , 
practical  applications,  or  were  merely  intended  to  give  a  general 
idea  of  the  composition  of  the  rock  in  question.  They  were  sel- 
dom employed  for  close  niineralogical  computations.  Those 
geologists  who  considered  the  subject  at  all,  believed  that  analyses 
were  so  variable  and  were  so  largely  a  function  of  the  sample 
taken,  that  they  might  differ  greatly  if  the  materials  were  derived 
merely  from  opposite  ends  of  a  hand-specimen.  They  therefore 
gave  them  comparatively  small  attention.  Even  when  analyses 
were  to  a  certain  extent  recast,  as  for  instance  in  the  Reports  of 
the  Survey  of  the  Fortieth  Parallel,  only  the  percentages  of  oxygen 
in  the  SiO2,  A12O3,  Fe2O3,  FeO,  etc.,  were  deducted  from  the  total 
percentages  of  the  oxides  and  were  used  to  calculate  the  so-called 
"  oxygen  ratio  " ;  that  is,  the  continued  ratio  of  the  percentage  of 
oxygen  combined  with  the  silicon,  to  that  combined  with  the 
monad  and  dyad  bases,  to  that  combined  with  the  triads.  The 
quotient  obtained  by  dividing  the  sum  of  the  last  two  by  the  first 
was  called  the  oxygen  quotient  and  was  esteemed  to  be  character- 
istic of  the  several  groups  of  igneous  rocks. 

The  following  ranges  of  oxygen  quotients  would  not  be  far  from 
the  truth.     It  is  well  to  add  that  the  higher  the  silica  the  lower  the 

*  This  chapter  in  practically  its  present  form  originally  appeared  in  the  School  of 
Mines  Quarterly,  November,  1900,  75. 

ii  161 


162  A   HAND  BOOK  OF  ROCKS. 

quotient     Ultra-basic  rocks  would  run  even  higher  than  the  values 

here  given. 

Rhyolites  —  Granites  .  1 7  5-.  3  50 

Trachytes  —  Syenites  .  3  5  o-.  5  7  5 

Dacites  —  Quartz-diorites  .275-.35O 

Andesites  —  Diorites  .  3  5  o-.  5  oo 

Basalt  —  Gabbro  .  54O-.6;  5 

To  a  certain  degree  these  values  are  characteristic,  and  being  in 
each  case  a  single  number  which  summarizes  a  whole  analysis 
they  are  more  easily  employed  than  are  the  equally  characteristic 
percentages  of  several  oxides,  but,  after  all  is  said,  the  contrasts 
are  based  upon  no  very  fundamental  or  at  least  no  very  definite 
principle,  and  they  give  no  clew  to  the  mineralogy  of  the  rock. 
The  same  quotients  may  be  obtained  with  widely  differing  aggre- 
gates of  minerals  and  from  very  dissimilar  rocks. 

From  the  introduction  of  microscopic  methods  of  investigation 
up  to  approximately  1890,  the  energies  of  practically  all  students 
of  the  subject  were  devoted  to  observing  and  recording  mineralog- 
ical  and  textural  details  and  the  subject  of  chemical  composition 
received  but  slight  attention.  It  was  revived,  however,  toward  the 
close  of  the  eighties  by  W.  C.  Brogger,  then  in  Stockholm,  and  in 
the  course  of  time  has  received  wide  recognition  and  employment 
from  many  others. 

Petrographers  are  now  accustomed  to  recast  an  ordinary  chem- 
ical analysis  by  dividing  the  several  percentages  by  the  molecular 
weights  of  the  corresponding  molecules,  so  as  to  obtain  a  series 
of  numbers,  which  are  called  the  "molecular  proportions"  or 
"  molecular  ratios."  These  quantities  indicate  the  relative  numbers 
of  the  several  molecules  in  the  rock  magma,  and  in  that  respect 
are  more  significant  than  are  the  percentages.  Using  the  molecular 
proportions  as  fundamentals,  curves  or  diagrams  of  various  sorts 
can  be  plotted,  which  will  indicate  in  a  graphic  way  the  variations 
in  composition  of  a  series  of  igneous  rocks  in  a  single  district,  or 
the  variations  in  a  single  family,  the  specimens  coming  from  various 
districts.  Many  interesting  conclusions  may  be  drawn  and  many 
characteristics  shown.  The  molecular  compositions  of  the  com- 
mon rock-making  minerals  are  now  quite  accurately  determined 


CHEMICAL  ANALYSES  OF  ROCKS.  163 

and  understood,  and  using  them  it  is  often  possible  to  calculate 
from  the  molecular  proportions  furnished  by  a  rock  analysis,  the 
percentages  of  the  several  minerals  in  the  rock.  The  calculations 
are  usually  checked  in  a  general  way  by  a  study  of  thin  sections. 

The  commoner  rock-making  minerals  and  their  molecular  com- 
positions are  given  below  in  the  tables,  to  which  reference  may  be 
made  in  following  the  accompanying  illustrations,  but  it  may  be 
remarked  that  petrographers  are  accustomed  to  regard  minerals 
of  complex  compositions  as  made  up  of  combinations  in  varying 
proportions  of  simple  molecules.  Thus  labradorite  is  a  lime-soda 
feldspar,  but  it  is  conceived  to  be  formed  by  a  combination  in  the 
proper  proportions  of  the  albite  molecule,  Na2O,Al2O3,6SiO2,  with 
the  anorthite  molecule  CaO,Al2O3,2SiO2.  Hypersthene  is  a  sili- 
cate of  magnesia  and  ferrous  oxide,  but  we  think  of  it  as  a  com- 
bination of  MgO,SiO2  with  FeO,SiO2.  Olivine  is  also  a  silicate 
of  magnesia  and  ferrous  oxide,  and  is  regarded  as  a  combination 
of  (MgO)2SiO2  with  (FeO)2SiO2. 

In  a  recent  joint  report  by  W.  H.  Weed  and  L.  V.  Pirsson,*  the 
latter  presents  recalculated  analyses  of  a  large  number  of  igneous 
rocks.  The  following  example  is  selected  from  pp.  466—467.  A 
syenite  was  gathered  at  the  Wright  and  Edwards  mine,  Barker, 
Mont.,  and  was  analyzed  by  W.  F.  Hillebrand  of  the  U.  S.  Geo- 
logical Survey.  A  number  of  minor  and  relatively  unimportant 
determinations  were  made,  in  addition  to  those  here  quoted,  as  for 
instance  TiO2,  H2O,  CO2,  BaO  and  SrO,  all  amounting  to  1.50.  In 
the  citation  below,  the  molecular  proportions  are  given  under  the 
respective  percentage  values. 

SiO,         A1,OS        Fe.O,       FeO         MgO       CaO       Na2O       K,O      P.O.        Cl  TotaL 

64.64         16.27         2.42        1.58        1.27        2.65        4.39       4.98        .37        .05          98.62 
1.077  'X58         .015        .022        .031        .047        .070       .053     .002     .0014 

From  an  examination  of  thin  sections  with  the  microscope  it 
was  observed  that  the  minerals  in  the  rock  were  the  following. 
The  quartz,,it  may  be  remarked,  was  inconspicuous,  so  that  the 
rock  is  called  a  syenite,  the  total  silica  being  at  the  same  time 
below  the  percentages  of  a  possible  feldspar,  albite. 

*  Geology  of  the  Little  Belt  Mountains,  Montana,  by  W.  H.  Weed,  with  a  report 
on  the  Petrography  by  L.  V.  Pirsson.  2oth  Ann.  Rep.  Dir.  U.  S.  Geol.  Survey,  III., 
257.  The  analysis  is  taken  from  p.  466. 


164 


A   HAND  BOOK  OF  ROCKS. 


6SJO, 


Orthoclase,  K,O,  A1,O,,  6SiO, 

.  f  Albite,  Na,0,  K 

PlagKKlase   j  ^^  ^ 

(MgO,  SiO, 
CaO,  SiO, 
FeO,  SiO, 

Magnetite,  Fe,Ov  FeO 
Quartz,  SiO,. 

From  an  inspection  of  these  formulas  it  is  evident  that  all  the 
K2O  is  in  the  orthoclase  ;  all  the  Na2O  is  in  the  albite  ;  all  the 
remaining  A12O3  is  in  the  anorthite  and  requires  an  equivalent 
number  of  molecules  of  CaO.  The  remaining  CaO  is  in  the  horn- 
blende and  apatite.  The  apatite  can  be  calculated  on  the  basis  of 
the  P2O5.  All  the  MgO  is  in  the  hornblende.  All  the  Fe2O3  is  in 
the  magnetite  and  an  equivalent  number  of  molecules  of  FeO  are 
required  by  it  The  remainder  of  the  FeO  is  in  the  hornblende. 
The  excess  of  SiO2  then  remains  for  the  quartz.  The  molecular 
proportions  are  hereafter  employed  as  whole  numbers. 


Orthoclase. 

Albite. 

Anorthite. 

Excess. 

K,0 

53 

none 

SO 

70 

none 

35 

12 

ALOj 

53 

70 

35 

none 

SiO, 

3»8 

420 

70 

269 

The  total  CaO  is  47  ;  CaO  in  anorthite  35  ;  therefore  of  the  CaO 
12  remain  for  the  apatite  and  hornblende.  The  expanded  formula 
for  apatite  is  pCaO,  CaCl2,  3P2O5,  but  from  this  expression  we  are 
never  to  infer  that  the  CaCl2  exists  as  such  in  the  mineral.  The 
Ca  in  the  CaCl2  has  been  weighed  as  CaO.  Having  therefore 
abstracted  the  necessary  CaO  for  the  apatite  the  residue  will  go  to 
the  hornblende  as  shown  in  the  next  tabulation,  which  also  em- 
braces all  the  remaining  minerals. 


Apatite. 

Hornblende. 

Magnetite. 

Quartz. 

K 

2 
6.7 

5-3 

OS 

3» 

7 

»5 

SiO, 

5-3 

3» 

7 

225.7 

Fe,O, 

15 

CHEMICAL  ANALYSES  OF  ROCKS.  165 

In  order  to  turn  these  results  into  percentages  of  the  minerals 
in  the  rock,  we  multiply  the  several  molecular  proportions  by  the 
respective  molecular  weights. 

Thus — Magnetite,        .015    Fe,Os  X  160  =  2.42 

.015     FeO    X    72=  i. 08  Total,     3.50 

Hornblende,      .031     MgO  X   40=1.24 

.031     SiO,    X    60=1.86 

.005     CaO    X    56=    .28 

.005     SiO,    X    60=    .30 

.007     FeO    X    72=   .50 

.007  SiO,  X  60=  .42  Total,  4.60 
Anorthite,  9.73 

Albite,  36.68 

Orthoclase,  29.46 

Quartz,  13.54 

Apatite,  i.io 

Grand  Total,  98.61 

In  order  to  raise  these  individual  percentages  so  that  they  will 
make  an  even  hundred,  they  should  each  be  increased  about  1.5 
per  cent. 

Magnetite,  3.55 

Apatite,  i.il 

Hornblende  4.66 

Anortbite,  9.86 

Albite,  37.22 

Orthoclase,  29.90 

Quartz,  13.70 


The  above  values  differ  slightly  from  those  obtained  by  Profes- 
sor Pirsson,  because  apatite  was  not  reckoned  by  him,  the  lime  being 
attributed  to  the  hornblende  and  anorthite. 

In  one  respect  the  numerical  labor  may  be  shortened.  Thus 
the  percentage  of  Orthoclase  is  .O53K2O  x  94  +  .OS3A12O3  x  102 
+  6  (.05 sSiOj)  x  60,  an  expression  which  may  be  factored  into 
.053  [  94  -f  102  -f  (6  x  60)  ].  This  latter  is  merely  the  molecular 
weight  of  Orthoclase  multiplied  by  the  molecular  proportion  of  the 
K2O,  the  oxide  which  gave  us  the  clue  to  the  original  calculation 
of  the  Orthoclase.  For  this  purpose  the  molecular  weights  of  the 
several  rock-making  minerals  are  later  given. 

If  the  albite  molecules  were  all  combined  with  the  anorthite  ones 
in  order  to  yield  a  plagioclase  —  the  relative  amounts  of  each  in 


166  A    HAND  BOOK  OF  ROCKS. 

the  plagioclase  would  be  proportional  to  the  sums-  of  the  molecular 
proportions  of  the  component  oxides,  as  given  in  the  tabulation, 
p.  152  ;  i.  e.t  albite,  70  -f  70  +  420  =560  and  anorthite  35  +  35 
4.  70  =  140.  This  would  be  Ab4An.*  But  some  of  the  albite  is 
in  the  orthoclase.  Pirsson  found  by  determination  of  the  optical 
properties  of  the  plagioclase  that  it  was  approximately  AbjAn. 
Half  the  albite  molecules  were  therefore  in  the  orthoclase  or 
present  in  microperthite.  From  these  deductions  we  can  calculate 
the  ratio  of  alkali-feldspar  to  soda-lime  feldspar,  viz.  : 

424  Or  -f-  280  Ab  ==  704  Alkali  feldspar. 
280  Ab  -f-  140  An  as.  420  Soda-lime  feldspar. 

This  ratio  704 : 420  is  almost  exactly  5:3.  One  can  readily 
appreciate  the  accuracy  with  which  a  result  of  this  character  will 
enable  us  to  classify  rocks  as  orthoclase  or  alkali  feldspar  rocks 
and  as  plagioclase  rocks. 

In  the  actual  performance  of  these  recalculations,  the  minera- 
logical  composition  of  the  rock  is  not  always  so  simple  as  in  the 
case  cited.  For  instance,  when  biotite  is  present  with  orthoclase, 
one  cannot  say  how  much  potash  and  alumina  belong  with  each  ; 
and  if  hornblende  is  also  present,  the  distribution  of  the  magnesia 
and  iron  oxide  presents  difficulties.  In  such  cases  it  may  be  neces- 
sary to  separate  and  analyze  one  of  the  minerals  in  order  to  furnish 
the  clue,  by  which  the  analysis  may  be  unraveled.  When,  at 
some  future  day  the  necessary  data  shall  have  been  accumulated, 
there  is  little  question  that  rocks  of  similar  textures  will  be  classi- 
fied and  defined  on  the  basis  of  their  percentages  of  the  several 
component  minerals. 

The  molecular  compositions  of  the  more  important  rock-making 
minerals  are  here  given  together  with  their  molecular  weights. 
Next  a  series  of  tables  similar  to  tables  of  logarithms  is  appended 
by  means  of  which  molecular  proportions  can  at  once  be  looked 
up  and  set  down  for  all  percentages  of  the  more  abundant  oxides 
which  are  likely  to  occur  in  the  usual  run  of  analyses.  It  is  hoped 
that  by  their  use  the  recalculation  of  analyses  may  be  facilitated 
and  more  often  performed. 

*  In  the  customary  abbreviations  Ab  means  albite,  An,  anorthite,  and  Or,  orthoclase, 
as  explained  on  page  5. 


CHEMICAL  ANALYSES  OF  ROCKS.  167 

Mineral.  Formula  Molecular  Weight. 

Quartz,                               SiO,  60 

Feldspars. 

Orthoclase,                          K,O,Al,O,,6SiO,  556 

Albite,                                    Na,O,AL,O,,6SiO,  524 

Anorthite,                           CaO.AljOj^SiO,  278 

Feldspathoids. 

Nephelite,                            Na,O,Al,O,,2SiO,  284 

Leucite,                               K,O,Al,OS)4SiO,  436 

Melllite,                                i2CaO,2Al,O,,9SiOa  1416 

Analcite,                              Na,O,AlJO.v4SiOJ  +  2H,O  440 

Sodalite,                                SCNajO.AljOj^SiO,]  +  2NaCl  969 

Haiiynite,  3[Na,O,Al2O,,2SiO,]  +  2CaSO4  1124 

Noselite,  3[Na,O,Al,Oj,2SiO,]+  2Na,SO4  1  136 

Micas. 

Muscovite,  (K,H),O,Al,Os>2SiO, 

Biotite,  2[(H,K),O,  (  AlFe),O,.2SiO,] 

2(Mg,Fe)0,Si01. 

Amphiboles  and  Pyrox-    MgO,SiO,  100 

enes  contain  in  the  dif-    FeO.SiO,  132 

ferent  varieties  differ-    CaO,SiO,  116 
ent  proportions  of  the    (MgFe^A^Fe^O^SiO, 

molecules  here  given:    Na.,O,Fe2O,,4SiO,  462 

Olivine                                 f(MgO),,SiO,  140 

Ohvine,                             l(FeO)vSiO,  204 

Magnetite,                            FeO,Fe2O,  232 

f9CaO,3P,06,CaCl,  1041 

aF,  1008 


In  the  group  consisting  of  haiiynite  and  noselite  (often  called  re- 
spectively haiiyne  and  nosean)  neither  mineral  occurs  pure,  of  the 
formula  given,  because  the  two  molecules  always  replace  each 
other  —  the  combination  rich  in  lime  being  called  haiiynite,  that 
rich  in  soda,  noselite.  In  cases  where  as  in  muscovite,  biotite  and 
one  of  the  pyroxenes,  two  elements,  such  as  K  and  H,  Al  and  Fe, 
or  Mg  and  Fe  replace  each  other  in  indefinite  amounts,  no  molec- 
ular weight  can  be  calculated.  We  must  then  assume  separate 
and  relatively  simple  molecules;  for  instance  in  muscovite  KjO, 
A12O3,  2SiO2  and  H2O,  Al2Oy  2SiO2.  It  does  not  follow  however 
that  these  are  known  in  nature. 

As  stated  on  the  previous  page,  when  the  same  oxide  is  present 
in  two  or  more  minerals,  as  for  instance  K2O  in  orthoclase,  biotite, 
and  muscovite,  or  MgO  in  pyroxene  and  biotite,  or  in  amphibole 
and  biotite,  we  may  find  difficulty  in  allotting  the  proper  amounts 
to  each.  Our  only  course  is  then  to  estimate  from  a  study  of  the 


i68  A  HAND  BOOK  OF  ROCKS. 

hand-specimen  or  thin  section  relative  proportions  of  these  miner- 
als in  the  rock  and  assume  a  partition  accordingly.  Where  these 
minerals  are  in  large  quantities  the  possible  error  is  objectionable, 
but  where  one  or  the  other  is  in  small  amount  the  error  is  not  a 
very  serious  matter.  In  all  such  cases  the  whole  method  of  recast- 
ing fails  because  we  have  too  many  unknown  quantities  and  too 
few  equations  for  solution.  Our  only  recourse  then  is  either  to 
separate  the  special  mineral  in  question  and  analyze  it,  or  else  to 
assume  standard  minerals  as  is  done  in  the  quantitative  scheme  of 
classification  outlined  below,  but  the  standard  minerals  themselves 
may  not  and  often  do  not  exist  in  the  rock. 

The  other  difficulty,  referred  to  in  the  second  paragraph  above, 
is  inevitable,  when  as  in  the  case  of  muscovite,  biotite,  the  pyrox- 
enes, amphiboles  and  olivine,  one  base  may  be  replaced  to  a  vary- 
ing degree  by  another  of  isomorphous  character,  as  K2O  and  H2O 
in  muscovite,  MgO  and  FeO  in  all  the  ferromagnesian  ones.  Then 
we  do  not  know  how  much  of  each  to  use.  We  must  in  conse- 
quence assume  some  ratio.  Efforts  are  being  made  by  the  sepa- 
ration and  analysis  of  the  minerals  which  create  these  difficulties 
to  discover  and  establish  certain  types  which  may  be  regarded  as 
fairly  characteristic  and  uniformly  present  in  rocks  of  related  chem- 
ical composition.  The  requisite  data  for  generalizations  are  hardly 
yet  at  command,  but  the  outlook  is  encouraging. 

In  the  recasting  of  basic  rocks  containing  both  pyroxene  and 
olivine,  or  amphibole  and  olivine,  we  first  calculate  tentatively,  as  if 
only  pyroxene  were  present.  If  there  is  then  too  little  SiO2  to 
satisfy  the  MgO  and  FeO  on  the  unisilicate  ratio,  it  is  necessary  to 
distribute  it  between  unisilicates  and  bisilicates,  the  latter  of  course 
taking  twice  the  bases  for  the  same  silica.  It  is  necessary  also  to 
assume  or  to  know  the  ratio  of  MgO:  FeO  in  both  unisilicates  and 
bisilicate.  This  being  true  and  letting 

a  =  FeO,  b  =  MgO,  c  =  SiO2  available,  all  being  known  quantities ; 
;r=the  silica  in  the  unisilicates,  being  also  equal  to  the  MgO  +  FeO 

in  the  same 
c  —  x  =  the  silica  in  the  bisilicates,  being  also  half  the  MgO  -f  FeO 

in  the  same. 
Then 

2(c  —  x)  -f  x  =  a  -f  b,     x=2c  —  (a  +  b) 


CHEMICAL   ANALYSES  OF  ROCKS.  169 

Having  now  the  distribution  of  the  silica,  we  can  assign  to  its  two 
portions,  our  bases  according  to  the  assumed  or  known  ratios. 

In  the  following  tables  the  oxides  are  arranged  in  the  order  sug- 
gested by  H.  S.  Washington  in  the  American  Journal  of  Science, 
July,  1900,  p.  59.  It  is  the  most  convenient  and  significant  one 
for  petrographers  and  it  emphasizes  the  most  important  features, 
even  if  it  separates  oxides  of  like  chemical  properties,  such  as  SiO2 
and  TiO2,  A12O3,  Fe2O3  and  Cr2O3,  etc.  In  using  the  table  the 
units  of  percentage  are  in  the  left  line,  the  decimals  then  follow 
horizontally  to  the  right  as  in  logarithms,  but  by  a  proper  use  of 
the  decimal  point  the  same  values  will  answer  for  tenths  or  multi- 
ples by  ten,  of  these  percentages. 

Following  the  tables  and  on  p.  176,  the  factors  are  summarized 
for  turning  molecular  proportions  into  percentages.  The  factors 
are  usually  the  molecular  weights  and  are  necessarily  so  whenever 
the  entire  formula  of  a  mineral  can  be  factored  into  a  whole-number 
multiple  of  any  one  component.  In  the  cases  of  melilite,  sodalite, 
hauynite,  noselite,  and  apatite  this  cannot  be  done.  In  each  of  these 
cases  the  formula  has  been  reduced  to  an  expression  involving  the 
key  oxide  as  a  unit,  and  the  corresponding  factor  has  been  calculated. 
Thus  melilite,  !2CaO,  2A12O3,  9SiO2  becomes  CaO,  JA12O3,  fSiO, 
giving  in  this  form  a  molecular  weight  of  1 1 8,  by  which  factor  the 
total  molecular  proportion  of  the  CaO  obtained  for  this  mineral  in 
recasting  may  be  immediately  multiplied  in  order  to  get  the  per- 
centage of  the  mineral  itself. 

In  a  similar  way  the  molecules  of  the  other  minerals  cited  have 
been  treated.  While  these  recalculations  are  simple  in  themselves 
yet  they  have  an  element  of  the  obscure  and  the  elusive.  Prob- 
ably all  teachers  have  discovered  the  ease  with  which  the  inex- 
perienced become  confused. 

Having  determined  the  percentages  by  weight  of  the  several 
component  minerals  in  a  rock,  it  is  sometimes  desirable  to  express 
their  percentages  by  volumes.  This  may  be  done  if  the  weight- 
percentage  of  each  mineral  is  divided  by  its  specific  gravity.  The 
several  quotients  must  then  be  added  up  to  a  total,  which  when 
divided  into  each  of  the  quotients  in  turn  will  give  its  volume-per- 
centage. In  such  recasting  it  is  somewhat  surprising  to  the  inex- 
perienced to  note  what  a  relatively  small  percentage  by  volume,  a 


i;o  A  HAND  BOOK  OF  ROCKS. 

relatively  high  percentage  by  weight  of  a  heavy  mineral  involves. 
Calculations  along  these  lines  are  sometimes  of  much  importance 
in  the  investigations  of  lean  or  disseminated  ores.  Thus  those 
lean  magnetites  which  can  only  be  treated  by  crushing  and  mag- 
netic concentration  can  be  expressed  in  terms  of  much  greater 
significance  than  the  mere  chemical  analyses.  While  ores  of  this 
type  only  involve  normal  rock-making  minerals,  yet  galena  and 
blende  in  limestone,  or  any  others  regarding  whose  composition 
and  specific  gravity  we  have  the  necessary  data  can  be  treated  in 
a  precisely  similar  way. 

When  the  recasting  of  analyses  can  be  carried  out  it  forms  a  good 
check  on  the  accuracy  of  the  analytical  work,  because  if  the  per- 
centage results  do  not  actually  or  approximately  afford  some  rea- 
sonable combination  of  minerals,  errors  have  obviously  been  made. 

By  a  series  of  assumptions  regarding  the  composition  of  some 
of  the  minerals  which  make  trouble  in  the  recasting,  Messrs. 
Cross,  Iddings,  Pirsson  and  Washington  have  developed  a  series 
of  so-called  "  standard  "  minerals  into  which  for  purposes  of  quan- 
titative classification  any  analysis  of  an  igneous  rock  can  be  broken 
up.  This  computed  mineralogical  aggregate  is  called  the  "  norm." 
It  may  or  may  not  differ  seriously  from  the  actual  composition 
called  the  "  mode,"  but  once  the  norm  is  calculated,  any  rock  of 
which  a  good  analysis  has  been  made,  can  be  quickly  placed  in  the 
quantitative  scheme  of  classification.  The  methods  and  the  scheme 
itself  are  too  complicated  to  be  described  here  and  reference  should 
be  made  to  the  original  works.*  While  admirably  adapted  for  the 
purposes  of  the  investigator  into  the  chemical  relations  of  rocks 
and  magmas,  yet  for  the  reasons  that  it  makes  texture  play  the 
most  subordinate  part  of  all  in  its  determinations ;  that  it  requires 
a  chemical  analysis  for  its  application  in  new  cases ;  and  that  it 
deals  with  assumed  and  often  non-existent  minerals  instead  of  those 
really  present ;  it  cannot  be  used  by  the  ordinary  observer  or  the 
field  geologist. 

*  Cross,  Iddings,  Pirsson  and  Washington,  Quantitative  Classification  of  Igneous 
Rocks,  Chicago,  1903.  H.  S.  Washington,  Chemical  Analyses  of  Igneous  Rocks, 
Professional  Paper  No.  14,  U.  S.  Geological  Survey.  J.  P.  Iddings,  Chemical  Composi- 
tion of  Igneous  Rocks,  expressed  by  means  of  diagrams,  etc.  Professional  Paper  No.  18, 
do.,  do.  G.  I.  Finlay,  The  calculation  of  the  norm  in  igneous  rocks.  Jour.  Geol., 
XVIII.,  58,  1910.  Best  guide  for  beginners. 


CHEMICAL  ANALYSES   OF  ROCKS. 


171 


Silica,  810,.  Holec.  Weight  60.  Log.  1.778151. 


30 

.500 

•501 

.503 

.505 

.506 

.508 

.510 

•5" 

•513 

•SIS 

3i 

.516 

.518 

•  520 

.521 

.523 

•525 

.526 

.528 

•53° 

.531 

32 

•533 

•535 

.536 

.538 

•540 

•  541 

•543 

•545 

•  546 

-548 

33 
34 

.550 
.566 

'III 

•553 
.570 

.555 
•571 

•556 
•573 

•558 
•575 

.560 
.576 

.561 
.578 

.563 
.580 

.565 
.581 

•583 

•585 

.586 

.588 

.590 

•591 

.593 

-595 

.596 

.598 

36 

.600 

.601 

.603 

.605 

.606 

.608 

.610 

.611 

.613 

.615 

37 

.616 

.618 

.620 

.621 

.623 

.625 

.626 

.628 

.630 

.631 

38 

•633 

•635 

.636 

.638 

.640 

.641 

.643 

.645 

.646 

.648 

39 

.650 

.651 

•653 

-655 

.656 

.658 

.660 

.661 

.663 

.665 

40 

.666 

.668 

.670 

.671 

•673 

.675 

.676 

.678 

.680 

.681 

•683 

.685 

.686 

.688 

.690 

.691 

-693 

.695 

.696 

.698 

42 

.700 

.701 

•703 

•705 

.706 

.708 

.710 

.711 

.713 

.715 

43 

.716 

.718 

.720 

.721 

.723 

•725 

.726 

.728 

.730 

•731 

44 

•733 

•735 

.736 

.738 

.740 

.741 

•  743 

.745 

.746 

•748 

9 

•75o 
.766 

:! 

-753 
.770 

•755 
.771 

•756 

•773 

•758 
•775 

.760 
.776 

.761 
.778 

.763 
.780 

:JS! 

47 
48 

.783 
.800 

.786 
.803 

.788 
.805 

.790 
.806 

•  791 
.808 

•793 
.810 

-795 
.811 

.796 
•  813 

f 

49 

.816 

!8i8 

.820 

.821 

.823 

.825 

.826 

.828 

.830 

So 

•833 

.835 

.836 

.838 

.840 

.841 

.843 

-845 

.846 

.848 

.851 

.853 

•855 

.856 

.858 

.860 

.861 

-863 

.865 

52 

.866 

.868 

.870 

.871 

•873 

.875 

.876 

.878 

.880 

.881 

53 

.883 

.885 

.886 

.888 

.890 

.891 

•893 

•895 

.896 

.898 

54 

.900 

.901 

.903 

.905 

.906 

.908 

.910 

.911 

•913 

.915 

55 

.916 

.918 

.920 

.921 

.923 

•925 

.926 

.928 

•930 

•931 

56 

•933 

•935 

•936 

.938 

.940 

.941 

•943 

•945 

.946 

.948 

57 

.951 

•953 

•955 

•956 

.958 

.960 

.96! 

•963 

.965 

58 

.966 

.968 

•970 

.971 

•973 

•975 

.976 

•978 

.980 

.981 

59 

•983 

•985 

.986 

.988 

.990 

.991 

•993 

•995 

.996 

.998 

60 

I.OOO 

I.OOI 

1.003 

1.005 

i.  006 

1.008 

1.  010 

i.  on 

1.013 

.015 

6l 

.016 

1.018 

i.  020 

I.O2I 

1.023 

1.025 

1.026 

1.028 

1.030 

.031 

62 

.033 

1-035 

1.036 

1.038 

1.040 

1.041 

.043 

1-045 

1.046 

.048 

63 

.050 

1-053 

1.055 

1.056 

1.058 

.060 

.061 

.063 

.065 

64 

.066 

i.  068 

1.070 

I.O7I 

1.073 

1.075 

.076 

.078 

.080 

.081 

65 

.083 

1.085 

i.  086 

1.088 

1.090 

1.091 

•093 

•095 

.096 

.098 

66 

.100 

I.IOI 

.103 

.105 

.106 

.108 

.110 

.in 

•"3 

•"5 

67 

.116 

1.118 

.120 

.121 

.123 

•  125 

.126 

.128 

.130 

68 

•133 

I-I35 

.136 

.138 

.140 

.141 

.143 

•145 

.146 

!HS 

69 

.150 

1.151 

•153 

•155 

.156 

.158 

.160 

.161 

.163 

.165 

70 

.166 

1.  1  68 

.170 

.171 

•173 

.175 

.176 

.178 

.180 

.181 

.183 

I.I85 

.186 

.188 

.190 

.191 

•193 

•195 

.196 

.198 

72 

.200 

I.2OI 

.203 

.205 

.206 

.208 

.210 

.211 

.213 

215 

73 

.216 

1.218 

.220 

.221 

.223 

.225 

.226 

.228 

.230 

.231 

74 

•233 

1-235 

.236 

.238 

.240 

.241 

•243 

.245 

.246 

.248 

.250 

I.25I 

•253 

•255 

.256 

.258 

.260 

.261 

.263 

.265 

p 

.266 

1.268 

.270 

.271 

'•273 

•275 

.276 

.278 

.280 

.281 

77 

.283 

1.285 

.286 

.288 

1.290 

.291 

-293 

.295 

.296 

.298 

78 

.300 

I.30I 

•303 

•3°5 

1.306 

.308 

.310 

1.311 

.315 

79 

.316 

.320 

.321 

1.323 

.325 

-326 

1.328 

1-330 

•331 

172 


A  HAND  BOOK  OF  ROCKS. 


Alumina,  AU)3,  Molec.  Weight  102.  Log.  2.00S600. 


g 

8 

_ 

o 

.000 

.001 

.002 

.003 

.004 

.005 

.006 

.007 

.008 

.009 

I 

.010 

.on 

.012 

.012 

.013 

.014 

.015 

.016 

.017 

.018 

2 

.020 

.021 

.021 

.022 

.023 

.024 

.025 

.026 

.027 

.028 

3 

.030 

.O3I 

.031 

.032 

•033 

•034 

•035 

.036 

•037 

.038 

4 

.040 

.041 

.041 

.042 

•043 

.044 

•045 

.046 

.047 

.048 

5 

.050 

.051 

.051 

.052 

•053 

•054 

•055 

.056 

•057 

.058 

6 

•059 

.060 

.061 

.062 

•063 

.064 

.065 

.066 

.067 

.068 

7 

.069 

.070 

.071 

.071 

.072 

•073 

.074 

•075 

.076 

.077 

8 

.078 

.080 

.080 

.O8l 

.082 

.083 

.084 

.085 

.086 

.087 

9 

.088 

.089 

.090 

.09! 

.092 

•093 

.094 

•095 

.096 

.097 

10 

.098 

.099 

.IOO 

.IOI 

.IO2 

.103 

.104 

.105 

.106 

.107 

ii 

.108 

.109 

.no 

.III 

.112 

.112 

•"3 

.115 

.116 

12 

.117 

.118 

.119 

.120 

.121 

.122 

.123 

.124 

•125 

.126 

13 

.127 

.128 

.129 

.130 

•I31 

.132 

•134 

•135 

.136 

M 

•137 

.138 

•139 

.140 

.141 

.142 

•143 

.144 

•  145 

.146 

!| 

.I47 

.148 

.149 

•J5i 

.I|2 

•J54 

.156 
166 

17 

•!57 
.167 

.168 

.170 

.170 

.171 

.172 

•173 

.174 

•175 

18 

.176 

.177 

. 

.179 

.180 

.181 

.182 

.183 

.184 

.185 

19 

.186 

.187 

.188 

.I89 

.190 

.191 

.192 

•193 

.194 

•195 

20 

.196 

.197 

.198 

.199 

.200 

.201 

.202 

.203 

.204 

.205 

21 

.206 

.207 

.208 

.209 

.210 

.211 

.212 

.213 

.214 

•215 

22 

.216 

.217 

.218 

.219 

.220 

.221 

.222 

.223 

.224 

.225 

23 

.226 

.227 

.228 

.220 

.230 

.230 

.231 

.232 

•233 

•  234 

24 

•  235 

.236 

•237 

.238 

•239 

.240 

.241 

.242 

•  243 

.244 

25 

.245 

.246 

.247 

.248 

.249 

.250 

.251 

.252 

•253 

26 

•255 

.256 

•257 

.258 

•259 

.260 

.261 

.262 

.263 

.264 

27 

.265 

.266 

.267 

.268 

.270 

.271 

.272 

•273 

.273 

28 
29 

:3J 

•275 
.285 

.276 
.286 

.277 
.287 

[288 

.279 
.289 

.290 

.281 
.291 

.282 
.292 

•293 

Ferric  Oxide,  Fe,0,,  Molec.  Weight  160.  Log.  2.204120. 


o 

I 

2 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

.000 

.001 

.002 

.002 

.003 

.003 

.004 

.005 

.005 

I 

.006 

.007 

.007 

.008 

.009 

.009 

.010 

.010 

.Oil 

.012 

2 

.013 

.013 

.014 

.014 

.015 

.015 

.016 

.017 

.017 

.018 

3 

.019 

.020 

.020 

.020 

.O2I 

.022 

.022 

.023 

.024 

.024 

4 

.025 

.025 

.026 

.027 

.027 

.028 

.029 

.029 

.030 

.030 

5 

.031 

.032 

.032 

•033 

•034 

•034 

•°35 

•035 

.036 

•037 

6 

•037 

.038 

.039 

.039 

.040 

.040 

.041 

.042 

.042 

•043 

7 

.044 

.044 

.045 

•045 

.046 

.047 

.048 

.049 

.049 

.050 

8 

.050 

.050 

.051 

.052 

.052 

•053 

.054 

.054 

•055 

•055 

9 

.056 

.057 

.057 

.058 

.059 

•°59 

.060 

.060 

.061 

.062 

10 

.062 

.063 

.064 

.064 

.065 

.065 

.066 

.067 

.067 

.068 

ii 

.069 

.069 

.070 

.070 

.071 

.072 

.072 

•073 

.074 

.074 

12 

»3 

3f 

•075 
.082 

.076 
.082 

.077 
.083 

•077 
.084 

.078 
.084 

.079 
.085 

.079 
.085 

.080 
.086 

.080 
.087 

»4 

.087 

.088 

.089 

.089 

.090 

.090 

.091 

.092 

.092 

•093 

IS 

.094 

.094 

.095 

•°95 

.096 

.097 

•097 

.098 

.099 

.099 

CHEMICAL  ANALYSES   OF  ROCKS. 


173 


Ferrous  Oxide,  FeO,  Molec.  Weight  72.  log.  1.857333. 


0 

i 

2 

3 

4 

5 

6 

7 

8 

9 

o 

.OOO 

.001 

.003 

.004 

.005 

.007 

.008 

.010 

.Oil 

.012 

I 

.014 

.015 

.017 

.'oil 

.019 

.021 

.022 

.024 

.025 

.027 

2 

.028 

.030 

.030 

.032 

•033 

•035 

.036 

.038 

•039 

.040 

3 

4 

.042 
.056 

•043 
•°57 

.044 
.058 

.046 
.060 

.048 
.061 

.049 
.062 

.050 
.064 

35 

'.067 

'.068 

5 

.070 

.071 

.072 

.074 

.075 

.076 

.078 

.079 

!o8o 

!o82 

6 

.083 

.085 

.086 

.088 

.089 

.090 

.092 

•093 

•094 

.096 

7 

.097 

.099 

.100 

.101 

.103 

.104 

.106 

.107 

.108 

.110 

8 

.III 

.112 

.114 

.115 

.117 

.118 

.120 

.121 

.122 

.124 

9 

.125 

.126 

.128 

.129 

.131 

•133 

•134 

.136 

•137 

.138 

10 

.140 

.140 

.141 

•143 

.144 

.146 

.147 

.149 

.150 

•151 

ii 

.154 

.156 

•157 

.158 

.160 

.161 

.162 

.164 

.165 

12 

167 

1  68 

.170 

.171 

.172 

•  174 

•'75 

.176 

.178 

.179 

13 

.'180 

.182 

.183 

.185 

.186 

.188 

.189 

.190 

.192 

•193 

14 

.194 

.196 

.197 

.199 

.200 

.201 

.203 

.204 

.206 

.207 

15 

.208 

.210 

.211 

.212 

.214 

.215 

.217 

.218 

.220 

.221 

Magnesia,  MgO,  Molec.  Weight  40.  Log.  1.602060. 


O 

i 

2 

3 

4 

5 

6 

7 

8 

9 

O 

.000 

.002 

.005 

.007 

.010 

.012 

.015 

.017 

.020 

.022 

I 

.025 

.027 

.030 

.032 

•035 

•037 

.040 

.042 

.045 

.047 

2 

3 

.050 

•075 

.052 
.077 

•055 
.080 

31 

.060 
.085 

.062 
.087 

.065 
.090 

.067 
.092 

.070 
.095 

.072 
.097 

4 

.100 

.102 

.105 

.107 

.110 

.112 

.115 

.117 

.120 

.122 

5 

.125 

.127 

.130 

.132 

.135 

•137 

.140 

.142 

.145 

.147 

6 

.150 

.152 

•155 

•157 

.160 

.162 

.165 

.167 

.170 

.172 

7 

.175 

.177 

.180 

.182 

.185 

.I87 

.190 

.192 

•195 

.I97 

8 

.200 

.202 

.205 

.207 

.210 

.212 

.215 

.217 

.220 

.222 

9 

.225 

.227 

.230 

.232 

•235 

•237 

.240 

.242 

•*45 

.247 

10 

.250 

.252 

•255 

•257 

.260 

.262 

.265 

.267 

.270 

.272 

ii 

.275 

.277 

.280 

.282 

.285 

.287 

.290 

.292 

.295 

.297 

12 

.300 

.302 

•305 

•307 

.310 

.312 

•315 

•3*7 

.320 

•  322 

13 
15 

•325 
.350 

•375 

•327 
•352 

•377 

•330 

•355 
.380 

•332 
•357 
.382 

•335 
.360 

.385 

is 

•387 

•  340 
.365 
•390 

•342 
•367 
•392 

•345 
•370 
•395 

•347 
•372 
•397 

16 

.400 

.402 

•405 

.407 

.410 

.412 

.415 

•417 

.420 

.422 

»7 

•425 

•427 

•430 

•432 

•435 

•437 

.440 

.442 

.445 

•447 

18 
19 

•450 
.475 

•452 
•477 

111 

.460 
.485 

.462 
•487 

.465 
.490 

.467 
.492 

.470 
•495 

•472 
•497 

20 

.500 

.502 

•505 

.507 

.510 

•  512 

•515 

.517 

•520 

.522 

21 

22 

•525 
•550 

•527 

•552 

•530 
•555 

•532 
•557 

jg 

•537 
.562 

•540 
.565 

•  542 
.567 

•545 
.570 

•547 
.572 

23 

24 

•575 
.600 

•577 
.602 

.580 
.605 

.582 

.585 
.610 

.587 
.612 

•590 
.615 

•592 
.617 

•595 
.620 

•597 
.622 

25 

.625 

.627 

.630 

^2 

•635 

•637 

.640 

.642 

.645 

•647 

174 


A  HAND  BOOK  OF  ROCKS. 


Lime,  CaO,  Holec.  Weight  56.  Log.  1.748188. 


g 

' 

9 

0 

.000 

.002 

.003 

.005 

.007 

.009 

.010 

.012 

.014 

.016 

I 

.018 

.020 

.O2I 

.023 

.025 

.027 

.029 

.030 

.032 

•034 

2 

.036 

.038 

•039 

.041 

•043 

•045 

.047 

.048 

.050 

.051 

3 

•053 

•055 

.057 

•059 

.060 

.062 

.064 

.066 

.068 

.070 

4 

.071 

•073 

•075 

.077 

.078 

.080 

.082 

.084 

.086 

.087 

5 

.089 

.091 

•093 

.094 

.096 

.098 

.100 

.IO2 

.103 

.105 

6 

.107 

.109 

.no 

.112 

."4 

,ri6 

.118 

.I2O 

.121 

•123 

7 

.125 

.127 

.128 

.130 

.132 

•134 

•135 

•137 

•139 

.141 

8 

.143 

.144 

.146 

.148 

•  153 

•IS5 

•157 

•159 

9 

.160 

.161 

.164 

.166 

.168 

.169 

.171 

•173 

•175 

•177 

10 

.178 

.180 

.182 

.184 

.185 

.187 

.189 

.191 

•193 

.194 

n 

.196 

.198 

.200 

.201 

.203 

.205 

.207 

.209 

.210 

.212 

12 

.214 

.216 

.218 

.219 

.221 

.223 

.225 

.226 

.228 

.230 

13 

.232 

.234 

•235 

•237 

•239 

.241 

•243 

.244 

.246 

.248 

*4 

.250 

.251 

•253 

•255 

•257 

.259 

.260 

.262 

.264 

.266 

Soda,  \a,0,  Molec.  Weight  62.  Log.  1.792392. 


o 

i 

2 

3 

4 

5 

6 

7 

8 

9 

O 

.000 

.001 

.003 

.005 

.006 

.008 

.009 

.Oil 

.013 

.014 

I 

.016 

.018 

.OI9 

.021 

.022 

.024 

.026 

.027 

.029 

.030 

2 

.032 

.034 

•035 

•037 

•039 

.040 

.042 

.043 

•045 

.047 

3 

.048 

.050 

.051 

•053 

•OSS 

.056 

.058 

•059 

.061 

.063 

4 

.064 

.066 

.068 

.069 

.071 

.072 

.074 

.076 

.077 

.079 

5 

.080 

.082 

.084 

.085 

.087 

.089 

.090 

.092 

•093 

.095 

6 

.097 

.098 

.100 

.101 

.103 

.105 

.106 

.108 

.III 

7 

•"3 

.114 

.116 

.118 

.119 

.121 

.122 

.124 

.126 

.127 

8 

.129 

.130 

.132 

•134 

•135 

•137 

•139 

.140 

.142 

•143 

9 

•145 

.147 

.148 

.150 

•151 

•153 

•155 

.156 

.158 

•159 

10 

.161 

.163 

.164 

.166 

.168 

.169 

.171 

.172 

•"74 

.176 

n 

.177 

.179 

.I80 

.182 

.184 

.I85 

.187 

.189 

.190 

.192 

12 

•  193 

.195 

.197 

.198 

.2OO 

.2OI 

.203 

.205 

.206 

.208 

13 

.209 

.211 

.212 

.214 

•215 

.217 

.219 

.221 

.222 

.224 

*4 

.226 

.227 

.229 

.230 

.232 

•234 

•235 

•237 

•239 

.240 

IS 

.242 

•243 

.245 

.247 

.248 

.250 

.251 

-253 

•255 

.256 

CHEMICAL  ANALYSES   OF  ROCKS. 


175 


Potash,  K,0,  Molec.  Weight  91.  Log.  1.973128. 


0 

i 

2 

3 

4 

5 

6 

7 

8 

9 

o 

.OOO 

.001 

.OO2 

.003 

.004 

.005 

.006 

.007 

.008 

.009 

I 

.OIO 

.012 

.013 

.014 

.015 

.016 

.017 

.018 

.019 

.020 

2 

.O2I 

.022 

.023 

.024 

.024 

.026 

.027 

.029 

.030 

.031 

3 

.032 

•Q33 

•034 

•035 

.036 

•037 

.038 

•039 

.040 

.041 

4 

.042 

•043 

.045 

.046 

.047 

.048 

.049 

•050 

.051 

•°53 

5 

•054 

•055 

.056 

•057 

•058 

•°59 

.060 

.061 

.062 

.063 

6 

.064 

.065 

.066 

.067 

.068 

.069 

.070 

.071 

.072 

•°73 

7 

.074 

•075 

.076 

.078 

.079 

.080 

.081 

.082 

.083 

.084 

8 

•  085 

.086 

.087 

.088 

.089 

.090 

.091 

.092 

•093 

•094 

9 

.096 

.097 

.098 

•099 

.100 

.101 

.102 

.103 

.104 

.105 

10 

.106 

.107 

.108 

.109 

.1X0 

.112 

•113 

.114 

.115 

.116 

n 

.117 

.118 

.119 

.120 

.121 

.122 

.123 

.124 

•125 

.126 

12 
13 

:3 

.129 
•139 

.130 
.140 

:\l\ 

.132 
.142 

•133 
•143 

•134 
.144 

•135 
.145 

.136 
.147 

•137 
.148 

.149 

.150 

.151 

•15* 

.153 

.154 

•155 

•  156 

.158 

IS 

•i59 

.160 

.162 

.163 

.164 

.I6S 

.166 

.167 

!i68 

.169 

Water,  11,0,  Holec.  Weight  18.  Log.  1.255273. 


o 

z 

2 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

•005 

.Oil 

.016 

.022 

.028 

•°33 

.040 

.044 

.050 

I 

•055 

.061 

.066 

.072 

.080 

•083 

.090 

.094 

.100 

•  105 

2 

.III 

.116 

.122 

.128 

•133 

•139 

.144 

.150 

•155 

.161 

3 

.166 

.172 

.178 

•  183 

.189 

.194 

.200 

.205 

.211 

.216 

4 

.222 

.228 

•233 

•239 

.244 

•  250 

•255 

.261 

.267 

.272 

.278 

.283 

.289 

.294 

.300 

•3°5 

•3" 

.316 

.328 

6 

•333 

•339 

•  344 

•350 

•355 

.361 

.367 

•372 

•378 

7 

.389 

•394 

.400 

•405 

.4" 

.417 

.422 

.428 

•433 

•439 

8 

•444 

.450 

•455 

.461 

•467 

.472 

•478 

•483 

.489 

•494 

9 

.500 

.505 

•5" 

•517 

•  522 

.528 

•533 

•539 

•544 

.550 

Carbonic  Acid,  CO,,  Molec.  Weight  44.  Log.  1.643453. 


o 

i 

2 

3 

4 

5 

6 

7 

8 

9 

o 

.000 

.002 

.004 

.007 

.009 

.on 

.014 

.016 

.018 

.020 

i 

2 

.023 
.045 

.025 
.048 

.027 
.050 

.029 
.052 

.032 
•055 

•034 
.057 

.036 

a? 

.041 
.064 

•043 
.066 

3 

.068 

.070 

•073 

.075 

.077 

.080 

.082 

.084 

.086 

.089 

4 

.091 

•093 

•095 

.098 

.100 

.102 

.104 

.107 

.109 

.III 

5 
6 

7 
8 
9 

^136 
•159 
.182 
.204 

.116 

J39 

'.54 

.207 

.118 
.141 

:5S 

.209 

.120 

•143 
.166 
.I89 
.211 

.123 

.191 
.2 

:53 

.170 

•193 
.216 

.127 
•  150 
•173 
•195 
.218 

•129 

.220 

.132 
.154 
.177 
.200 
•  223 

•134 
•157 
.179 
.202 
.225 

I76  A   HAND  BOOK  OF  ROCKS. 

Titanic  Acid,  HO*  Molec.  Weight  82.  Log.  1.913814. 


o 

.000 

.001 

.002 

.003 

.005 

.006 

.007 

.008 

.010 

.Oil 

I 

.012 

.013 

.014 

.015 

.017 

.018 

.019 

.020 

.022 

.023 

2 

.024 

.025 

.026 

.028 

.029 

.030 

.031 

•033 

•034 

•°3S 

3 

.036 

.038 

.039 

.040 

.041 

.043 

.044 

•045 

.046 

.047 

4 
5 

.049 

.061 

.050 
.062 

.051 
.063 

3 

:°JI 

58 

.056 
.068 

.057 
.069 

.058 
.070 

•059 
.072 

6 

.073 

.074 

.075 

.077 

.078 

.079 

.080 

.081 

•083 

.084 

7 

.081 

.086 

.088 

.089 

.090 

.091 

.092 

.094 

.095 

.096 

8 

•097 

.098 

.100 

.101 

.102 

.103 

.105 

.106 

.107 

.108 

9 

.109 

.III 

.112 

•"3 

.114 

.116 

.117 

.118 

.119 

.120 

Zirconia,  Zr02,  Molec.  Weight  122.  Log.  2.086360. 


o 

I 

2 

3 

0 

.000 

.001 

.001 

.002 

.003 

.004 

.005 

.006 

.006 

.007 

Phosphoric  pentoxlde,  P205,  Molec.  Weight  142.  Log.  2.152288. 


O 

I 

2 

3 

0 

.000 

.000 

.001 

.002 

.003 

.003 

.004 

.005 

.005 

.006 

I 

.007 

.008 

.008 

.009 

.010 

.010 

.on 

.012 

.013 

.013 

2 

.014 

.015 

.015 

.016 

.017 

.017 

.018 

.019 

.020 

.020 

Sulphuric  anhydride,  SO,,  Molec.  Weight  80.  Log.  1.903090. 


0 

I 

2 

3 

4 

5 

6 

7 

8 

9 

o 
I 

.000 
.012 

.001 

.014 

.002 
.015 

.004 
.016 

.005 

.017 

.006 
.019 

.007 
.020 

.009 

.021 

.010 
.022 

.Oil 

.024 

Chlorine,  €1,  Atomic  Weight  35.5.  Log.  1.550228. 


0 

I 

2 

3 

4 

5 

6 

7 

8 

9 

o 

.000 

.002 

.006 

.008 

.Oil 

.014 

.017 

.020 

.022 

.025 

Fluorine,  F,  Atomic  Weight  19.  Log.  1.278754. 


0 

I 

a 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

.005 

.010 

.016 

.021 

.026 

.031 

•O37 

.042 

.047 

Sulphur,  8,  Atomic  Weight  32.  Log.  1.505150. 


O 

I 

2 

3 

4 

5 

6 

7 

8 

9 

o 

I 

2 

.OOO 

.003 

.006 

•037 
.069 

.009 
.040 
.072 

.012 
.044 
.075 

.016 

.047 
.078 

.019 
.050 
.081 

.022 

•053 
.084 

.025 

.056 
.087 

.028 
.059 
.091 

CHEMICAL  ANALYSES  OF  ROCKS. 


177 


Chromic  oxide,  Cr20,,  Molec.  Weight  152.8.  log.  2.184123. 


o 

z 

2 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

.000 

.001 

.002 

.002 

.003 

.004 

.004 

.005 

.006 

I 

.006 

.007 

.008 

.008 

.009 

.010 

.010 

.Oil 

.012 

.012 

2 

.013 

.014 

.014 

.015 

.016 

.016 

.017 

.018 

.018 

.019 

3 

.020 

.020 

.021 

.022 

.022 

.023 

.024 

.024 

.025 

.026 

4 

.026 

.027 

.028 

.028 

.029 

.030 

.030 

.031 

.032 

.032 

5 

•033 

•034 

•034 

•035 

.036 

.036 

.037 

.038 

.038 

•039 

Nickel  oxide,  MO,  Molec.  Weight  15.  Log.  1.875061. 
Cobalt  oxide,  CoO,  Molec.  Weight  75.  Log.  1.875061. 


0 

z 

2 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

.001 

.002 

.004 

.005 

.006 

.008 

.010 

.Oil 

.012 

I 

.013 

.015 

.016 

.017 

.019 

.020 

.021 

.023 

.024 

.025 

2 

.027 

.028 

.030 

.031 

.032 

•033 

•035 

.036 

•°37 

•039 

tupric  oxide,  CnO,  Molec.  Weight  79.1.  Log.  1.898176. 


o 

I 

2 

3 

4 

5 

6 

7 

8 

9 

o 

.000 

.001 

.OO2 

.004 

.005 

.006 

.007 

.009 

.010 

.on 

Manganons  oxide,  MnO,  Molec.  Weight  71.  Log.  1.851258. 


0 

i 

2 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

.001 

.003 

.004 

.005 

.007 

.008 

.010 

.Oil 

.012 

I 

.014 

.015 

.017 

.018 

.020 

.021 

.022 

.024 

.025 

.027 

2 

.028 

.030 

.031 

.032 

•034 

•035 

.036 

.038 

•039 

.041 

3 

.042 

•043 

.045 

.046 

.048 

•049 

.050 

.052 

•053 

•OSS 

4 

.os6 

.oS8 

•059 

.060 

.062 

.061 

.o6s 

.066 

.068 

.O6q 

5 

.070 

.072 

•073 

.074 

.076 

.077 

.079 

.080 

.081 

.083 

Baryta,  BaO,  Molec.  Weight  152.8.  Log.  2.184123. 


0 

I 

2 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

.006 

.000 

.007 

.001 

.008 

.002 
.008 

.002 
.009 

.003 

.010 

.004 
.010 

.004 
.on 

.005 
.012 

.006 
.012 

Strontia,  SrO,  Molec.  Weight  103.5.  Log.  2.014940. 


o 

I 

2 

3 

4 

5 

6 

7 

8 

9 

0 

.000 

.000 

.OO2 

.003 

.004 

.005 

.006 

.007 

.008 

.009 

I 

.010 

.Oil 

.on 

.012 

.013 

.014 

•015 

.016 

.017 

.018 

Llthla,  Li,0,  Molec.  Weight  30.  Log.  1.477121. 


0 

I 

2 

3 

4 

0 

I 

.000 

•033 

.003 
.036 

.006 

.040 

.010 

•043 

.013 

.046 

.016 
.050 

.020 
•053 

31 

.026 
.060 

.030 
.063 

I78 


A   HAND  BOOK  OF  ROCKS. 


FACTORS   FOR  TURNING   MOLECULAR   PROPOR- 
TIONS  INTO   PERCENTAGES. 

In  each  case  multiply  the  molecular  proportion  of  the  given  constituent  oxide  or 
oxides  by  factor  cited  below,  which  is  usually  the  molecular  weight  of  the  compound. 
(For  explanation  see  p.  169.) 

Mineral. 

Composition. 

Oxide  Used. 

Factor. 

Quartz, 

SiO, 

SiO, 

60 

OrtAOClase, 

K,0,Al203,6SiO, 

K20 

556 

Albite, 

Na,0,Al2Os,6Si08 

Na,0 

524 

Anorthite, 

CaO,Al2O8,2SiO, 

CaO 

278 

Nephelite, 

Na10,AlJ08,2SiO, 

Na,0 

284 

Leucite, 

K,0,Al208,4SiOJ 

K20 

436 

Melilite, 

i2CaO,2Al2Oj,9SiO, 

CaO 

118 

Analcite, 

Na,O,Al2O8,4SiO22H,O 

Na,0 

440 

Sodalite, 

3(Na,0,Al,Os,2Si02)  +  2NaCl 

Na,0 

323 

Hauynite, 

3(Na,0,Al,Ov2S502)  +  2CaSO4 

NajO 

375 

Noselite, 

3(Na,0,Al,08,2Si02)  +2Na,S04 

Na,0 

379 

Muscovite, 

K20,Al20,,2Si02 
H,0,Al20,,2SiO, 

K20 
H20 

3i6 
240 

Biotite, 

2(K,0,Al20,,2Si02) 
2(K20,Fe20,,2Si02) 
2(H20,Al208>2Si02) 
2(H10,Fe208,2SiOl) 
2MgO,SiO. 
2FeO,SiO, 

K20 
KO 
HO 
HO 
SiO, 
SiO, 

3i6 
374 
240 
298 
140 
204 

AmphibolM  and 
Pyroxenes, 

MgO.SiO, 
FeO.SiO, 
CaO.SiO, 
MnO.SiO. 
MgO,Al2O,,SiO2 
MgO,Fe,Os,SiO, 
FeO,Al2O.,SiO2 
Na20,Fes68,4Si04 

MgO 
FeO 
CaO 
MnO 
MgO 
MgO 
FeO 
Na,O 

100 

132 
116 

I3« 

202 
260 
2J4 
462 

OliYine, 

2MgO,SiO, 
2FeO,SiO, 

Si02 
SiO, 

140 
204 

Magnetite, 

FeO,Fe2O, 

Fe,0, 

232 

Ilmenite, 

FeO.TiO, 

TiO, 

154 

Apatite, 

pCaO.aP^Caa, 
gCaO^PjO^CaF, 

CaO 
CaO 

116 
112 

Kaolin, 

Al20,,2Si02,2H20, 

Al,08 

258 

Serpentine, 

3MgO,2SiO,,2H,O 

MgO 

92 

Caldte, 

C«O,C02 

CaO 

100 

Magneaite, 

MgO,CO, 

MgO 

84 

CHEMICAL   ANALYSES   OF  ROCKS.  179 


Mineral. 

Composition. 

Oxide  Used. 

Factor. 

Titanite, 

CaO,TiO,,SiO 

TiO, 

I98 

Garnet, 

GroMularite, 

3CaO,Al,O3,3SiO, 

A1,O3 

450 

Almandite, 

SFeO.A^O^SiO, 

Al,03 

498 

Andradite, 

3CaO,Al,O3,3SiO, 

F«^03 

508 

Chromite, 

FeO.CrjO, 

Cr,03 

224.8 

Chlorite, 

2H,0,  2MgO,  A1,O,,  SiO, 

H20 

139 

2H20,2MgO,Fe103,SiO, 

H,0 

I48 

GLOSSARY. 


NOTE.  —In  the  following  definitions,  when  fuller  explanations  are  to  be  found  in 
preceding  pages,  references  are  given  to  them  and  they  should  be  consulted.  No 
attempt  has  been  made  to  unnecessarily  repeat  previous  statements. 


Aa,  a  Hawaian  word  specially  introduced  into  American  usage  by 
Maj.  C.  E.  Button,  and  employed  to  describe  jagged,  scoriaceous,  lava 
flows.  It  is  contrasted  with  pahoehoe.  4th  Ann.  Rep.  U.  S.  Geol. 
Survey,  95. 

Ablation,  a  name  applied  to  the  process  whereby  residual  deposits 
are  formed  by  the  washing  away  of  loose  or  soluble  materials. 

Absarokite,  a  general  name  given  by  Iddings  to  a  group  of  igneous 
rocks  in  the  Absaroka  range,  in  the  eastern  portion  of  the  Yellowstone 
Park.  They  have  porphyritic  texture  with  phenocrysts  of  olivine  and 
augite  in  a  groundmass,  either  glassy  or  containing  leucite,  orthoclase  or 
plagioclase,  one  or  several.  They  range  chemically,  SiO,,  46-52  ; 
A1,O,,  9-12  J  MgO,  8-13  ;  alkalies,  5-6.3,  with  potash  in  excess.  The 
name  is  of  greatest  significance  when  taken  in  connection  with  shosho- 
nite  and  banakite.  Jour,  of  Geol.,  III.,  936. 

Abyssal-rocks,  a  synonym  of  plutonic  rocks  as  used  in  preceding 
pages.  The  word  has  been  suggested  and  especially  used  by  W.  C. 
Brogger. 

Accessory  components  or  minerals  in  rocks  are  those  of  minor  im- 
portance or  of  rare  occurrence,  whose  presence  is  not  called  for  by  the 
definition  of  the  species. 

Acidic,  a  descriptive  term  applied  to  those  igneous  rocks  that  con- 
tain more  than  65  per  cent.  SiO^,  as  contrasted  with  the  medium  of  65 
per  cent,  to  55  per  cent,  and  the  basic  at  less  that  55  per  cent.  ;  still 
the  limits  are  somewhat  elastic. 

Acmite-trachyte,  a  trachyte  whose  pyroxene  is  acmite  or  segirite 
and  whose  feldspar  is  anorthoclase.  It  therefore  differs  from  normal 
trachyte  in  its  prevailing  soda  instead  of  potash,  as  is  shown  by  the 
acmite,  a  soda-pyroxene,  and  the  anorthoclase,  a  soda-feldspar.  The 
acmite-trachytes  are  intermediate  between  the  true  trachytes  and  the 

1 80 


GLOSSARY.  181 

phonolites.  They  were  first  described  from  the  Azores  (Mugge,  Neues 
Jahrbuch,  1883,  II.,  189)  and  have  also  been  found  in  the  Crazy 
Mountains,  Mont.;  see  p.  40,  Anals.  4  and  5. 

Adamellite,  a  name  proposed  by  Cathrein  as  a  substitute  for  tonalite, 
on  the  ground  that  tonalite  means  a  hornblende-biotite  granite,  rich  in 
plagioclase,  whereas  adamellite,  which  better  describes  the  rocks  at  the 
Tyrolese  locality,  means  a  quartz-hornblende-mica-diorite  with  granitic 
affinities.  Adamellite  emphasizes  the  dioritic  characters;  tonalite,  the 
granitic.  The  name  is  derived  from  Monte  Adamello,  near  Meran, 
Tyrol,  the  locality  of  tonalite.  Neues  Jahrb.,  1890,  I.,  75.  Brogger 
uses  it  for  acidic  quartz-monzonite.  Eruptions-folge  bei  Predazzo,  61. 

Adinole,  a  name  for  dense  felsitic  rocks,  composed  chiefly  of  an 
aggregate  of  excessively  fine  quartz  and  albite  crystals,  such  that  on 
analysis  the  percentage  of  soda  may  reach  10.  Actinolite  and  other 
minerals  are  subordinate.  Adinoles  occur  as  contact  rocks,  associated 
with  diabase  intrusions  and  are  produced  by  them  from  schists  (com- 
pare spilosite  and  desmite).  They  also  constitute  individual  beds  in 
metamorphic  series.  (Compare  porphyroid,  halleflinta.)  The  name 
was  first  given  by  Beudant,  but  has  been  especially  revived  by  Lossen. 
Zeits.  d.  d.  Geol.  Ges.,  XIX.,  572,  1867. 

Adobe,  a  word  widely  employed  in  Spanish  America  and  neighboring 
regions  for  the  argillaceous,  residual  soils  or  other  clayey  deposits  of 
the  arid  countries,  from  which  the  sun-baked  bricks  are  made. 

JEginte,  the  name  of  this  soda  pyroxene  is  often  prefixed  to  normal 
rock-names  because  of  its  presence,  as  for  instance,  aegirite-granite, 
aegirite-trachyte.  Microscopic  study  has  shown  that  the  mineral  is 
much  more  widely  distributed  than  was  formerly  appreciated. 

Aerolite,  a  synonym  of  meteorite. 

Agglomerate,  a  special  name  for  volcanic  breccias  as  distinguished 
from  other  breccias  and  from  conglomerates. 

Ailsyte,  a  name  derived  from  Ailsa  Craig,  Scotland,  and  suggested 
for  a  micro-granite  with  considerable  riebeckite,  which  occurs  there. 
M.  F.  Heddle,  Trans.  Edinburgh  Geol.  Soc.,  VII.,  265,  1897. 

Akerite,  a  special  name  coined  by  Brogger  for  a  variety  of  syenite  at 
Aker,  Norway,  that  is  a  granitoid  rock  consisting  of  orthoclase,  con- 
siderable plagioclase,  biotite,  augite  and  some  quartz.  (W.  C.  Brogger, 
Zeitsch.  f.  Krys.,  1890,  43.) 

Alaskite,  a  name  proposed  by  J.  E.  Spurr  as  a  general  term  for  all 
rocks  consisting  essentially  of  quartz  and  alkali  feldspar,  without  regard 
to  texture.  Those  with  xenomorphic  or  hypautomorphic  textures  are 
alaskites  proper;  those  with  panautomorphic  textures  alaskite-aplite ; 


1 82  A  HAND  BOOK  OF  ROCKS. 

those  with  porphyritic  texture  involving  a  fine-grained  or  aphanitic 
groundmass,  tordrillites  (which  see).  Amer.  Geol.,  XXV.,  231,  2Oth 
Ann.  Rep.  U.  S.  G.  S.  Part  7,  189,  195. 

Albite,  the  name  of  the  mineral  is  sometimes  prefixed  to  normal  rock 
names,  because  of  its  presence  in  the  rocks:  as  for  instance  albite- 
diorite,  albite-porphyrite. 

Albitite,  a  name  applied  by  H.  W.  Turner  to  granitoid  rocks,  con- 
sisting essentially  of  albite.  The  original  occurrence  is  a  series  of  dikes 
cutting  serpentine,  near  Meadow  valley,  Plumas  Co.,  Calif.,  but  under 
the  name  soda-syenite,  similar  rocks  have  been  described  from  various 
places  on  the  Pacific  coast.  (Amer.  Geologist,  June,  1896,  378-380.) 

Albitophyre,  a  name  given  by  A.  Michel-Levy  to  a  dike  rock,  in 
which  are  developed  very  large,  polysynthetic  phenocrysts  of  albite.  In 
the  groundmass  are  microlites  of  the  same  mineral,  together  with  chlorite 
and  limonite.  Comptes  rendus,  CXXII.,  265,  1896. 

Alboranite,  a  variety  of  hypersthene-andesite,  poor  in  soda,  from  the 
island  of  Alboran,  east  of  the  Straits  of  Gibralter,  and  80  km.  south 
from  Spain.  The  recasting  of  a  typical  analysis  gave  plagioclase, 
(Abi  An4.6),  41.5;  hypersthene,  5 ;  augite,  20;  magnetite,  9;  basis,  24.5; 
total,  100.  The  rocks  are  porphyritic  with  plagioclase  phenocrysts. 
F.  Becke,  Tschermaks  Mittheilungen,  XVIII.,  553,  1899. 

Aleutite,  a  name  proposed  by  J.  E.  Spurr  for  those  members  of  his 
belugites  (which  see)  having  a  porphyritic  texture  with  an  aphanitic  or 
finely  crystalline  groundmass.  Amer.  Geol.,  XXV.,  233,  1900.  2Oth 
Ann.  Rep.  U.  S.  G.  S.,  Part  7,  209. 

Algovite,  a  name  proposed  by  Winkler,  for  a  group  of  rocks,  practi- 
cally diabases,  or  porphyritic  phases  of  the  same,  in  the  Algauer  Alps. 
They  also  embrace  gabbros  according  to  Roth,  and  are  doubtless  various 
textural  varieties  of  an  augite-plagioclase  magma.  Neues  Jahrbuch, 
1895,  641. 

Allalinite,  a  name  derived  from  Allalin  mountain  in  the  Pennine 
Alps,  and  applied  by  H.  Rosenbusch  to  an  actinolite-saussurite  rock, 
which  had  been  derived  from  gabbro  without  losing  the  characteristic 
texture  of  the  latter.  That  is,  the  allalinites  are  not  sheared  and  crushed 
as  in  the  flaser-gabbros  and  forellensteins.  Massige  Gest.,  328,  1895. 

Allivalite,  Marker's  name  for  the  granitoid,  anorthite-olivine  rocks 
with  the  two  minerals  in  equal  amounts  or  with  the  feldspar  in  excess — 
small  proportions  of  other  minerals  such  as  augite  may  be  present. 
"Geology  of  the  Small  Isles."  Mem.  British  Geol.  Survey,  69-77,  I9°8- 

Allochetite,  a  name  derived  from  the  Allochet  valley  on  the  eastern 
side  of  the  Monzoni  region  of  the  Austrian  Tyrol  and  applied  by  J.  A. 


GLOSSARY.  183 

Ippen  to  a  dike  rock  related  to  the  tinguaites.  The  allochetite  is  por- 
phyritic  in  texture  and  contains  phenocrysts  of  plagioclase  (labradorite), 
orthoclase,  titanaugite,  nephelite  and  magnetite,  in  a  groundmass  of 
augite,  magnetite,  hornblende,  nephelite,  orthoclase  and  occasional 
biotite.  (Verh.  der.  k.  k.  geol.  Reichsanstalt,  Vienna,  1903,  132.) 

Allogenic,  originating  elsewhere;  whether  applied  to  the  components 
of  a  clastic  rock  or  to  xenoliths.  The  contrasted  term  is  authigenic. 

Allotriomorphic,  an  adjective  coined  by  Rosenbusch  in  1887  to 
describe  those  minerals  in  an  igneous  rock  which  do  not  possess  their 
own  crystal  faces  or  boundaries,  but  which  have  their  outlines  impressed 
upon  them  by  their  neighbors.  They  result  when  a  number  of  minerals 
crystallize  at  once  so  as  to  interfere  with  one  another.  They  are  espe- 
cially characteristic  of  granitoid  textures.  The  word  was  unnecessary, 
as  xenomorphic  had  been  earlier  suggested  for  the  same  thing,  but  it  is 
in  more  general  use  than  xenomorphic.  See  also  anhedron. 

Alluvium,  Lyell's  name  for  the  deposit  of  loose  gravel,  sand  and  mud 
that  usually  intervenes  in  every  district  between  the  superficial  covering 
of  vegetable  mould  and  the  subjacent  rock.  The  name  is  derived  from 
the  Latin  word  for  an  inundation  (Elements  of  Geol.,  6th  Ed.,  N.  Y., 
1859,  p.  79).  It  was  employed  by  Naumann  as  a  general  term  for  sedi- 
ments in  water  as  contrasted  with  eolian  rocks.  It  is  generally  used 
to-day  for  "the  earthy  deposit  made  by  running  streams  or  lakes,  espe- 
cially during  times  of  flood.  (Dana's  Manual,  1895,  p.  81.)  In  a 
stratigraphical  sense  it  was  formerly  employed  for  the  more  recent 
water-sorted  sediments,  as  contrasted  with  diluvium,"  or  the  stratified 
and  unstratified  deposits  from  the  melting  of  the  continental  glacier  of 
the  Glacial  Period.  This  use,  with  fuller  study  of  the  Glacial  deposits,  is 
practically  obsolete. 

Alnoite,  a  very  rare  rock  with  the  composition  of  a  melilite  basalt, 
that  was  first  discovered  in  dikes  on  the  island  of  Alno,  off  the  coast  of 
eastern  Sweden.  The  special  name  was  given  it  by  Rosenbusch  to  em- 
phasize its  occurrence  in  dikes  and  its  association  as  a  very  basic  rock, 
with  nepheline  syenite.  A'noite  has  been  discovered  near  Montreal  by 
F.  D.  Adams  (Amer.  Jour.  Sci.,  April,  1892,  p.  269)  and  at  Man- 
heim  Bridge,  N.  Y.,  by  C.  H.  Smyth,  Jr.  (Amer.  Jour.  Sci.,  Aug.,  1893, 
104). 

Alsbachite,  a  name  given  by  Chelius  to  a  variety  of  granite-porphyry, 
forming  dykes  in  Mt.  Melibocus,  and  containing  large  mica  crystals  and 
rose-red  garnets.  Notizbl.  Ver.  Erdk.  zu  Darmstadt,  1892,  Heft  13,  I. 

Alum-shales,  shales  charged  with  alum,  which  in  favorable  localities 
may  be  commercially  leached  out  and  crystallized.  The  alum  results 


1 84  A  HAND  BOOK  OF  ROCKS. 

from  the  decomposition  of  pyrites,  because  the  sulphuric  acid,  thus 
produced,  reacts  on  the  alumina  present,  yielding  the  double  sulphate 
that  is  alum. 

Ampelite,  a  name,  specially  current  among  the  French,  for  shales, 
charged  with  pyrite  and  carbonaceous  matter,  which  may  yield  alum- 
shales. 

Amphibole,  the  generic  name  for  the  group  of  bisilicate  minerals 
whose  chief  rock-making  member  is  hornblende.  It  is  often  prefixed  to 
those  rocks  which  have  hornblende  as  a  prominent  constituent,  as 
amphibole-andesite,  amphibole-gabbro,  amphibole-granite,  etc. 

Amphibolite,  a  metamorphic  rock  consisting  chiefly  of  hornblende, 
or  of  some  member  of  the  amphibole  group.  It  is  as  a  rule  a  synonym 
of  hornblende-schists,  but  is  preferable  to  the  latter,  when  the  schistosity 
is  not  marked.  See  p.  140. 

Amygdaloids  are  cellular  lavas,  whose  cavities,  caused  by  expanding 
steam-bubbles,  resemble  an  almond  in  size  and  shape.  Basaltic  rocks 
are  most  prone  to  develop  them.  The  term  is  used  in  the  form  of  the 
adjective,  amygdaloidal,  and  properly  should  be  limited  to  this.  As 
a  noun  it  is  also  employed  for  secondary  fillings  of  the  cavities,  which  are 
usually  calcite,  quartz  or  some  member  of  the  zeolite  group.  Amygda- 
loidal rocks  are  of  chief  interest  in  America,  because  certain  basaltic 
lava  sheets  on  Keweenaw  Point,  Lake  Superior,  have  their  amygdules 
filled  with  native  copper  and  are  important  sources  of  the  metal. 
Amygdaloidal  cavities  are  limited  to  the  upper  and  lower  portions  of 
lava  sheets.  The  name  is  derived  from  the  Greek  word  for  almond. 

Analcite-basalt,  a  variety  of  basalt  whose  feldspar  is  more  or  less  re- 
placed by  analcite.  The  analcite  is  at  times  in  such  relations  as  to  give 
reason  for  thinking  it  an  original  mineral  and  not  an  alteration  product 
from  feldspar.  Analcite-basalts  occur  in  the  Highwood  Mountains, 
Mont.  (See  W.  Lindgren,  loth  Census,  XV.,  727,  Proc.  Calif.  Acad. 
Sci.,  Ser.  II.,  Vol.  III.,  p.  51  Comptes  Rendus,  Fifth  Internat.  GeoK 
Cong.,  364.)  Analcite-diabase  has  been  met  in  California.  (H.  W. 
Fairbanks,  Bull.  Dept.  Geol.  Univ.  of  Calif.,  I.,  173.)  See  also  in 
this  connection  teschenite. 

Analcite-tinguaite,  tinguaite  (which  see)  with  considerable  analcite, 

Analcitite,  Pirsson's  name  for  the  olivine-free  analcite-basalts.  Jour. 
Geol.,  IV.,  690,  1896. 

Anamesite,  an  old  name  suggested  by  von  Leonhard,  in  1832,  for 
those  finely  crystalline  basalts,  which  texturally  stand  between  the 
dense  typical  basalt,  and  the  coarser  dolerites.  The  name  is  from  the 
Greek  for  "in  the  middle." 


GLOSSARY.  185 

Anamorphic,  a  term  coined  by  C.  R.  Van  Rise  in  discussing  the 
processes  of  metamorphism,  for  the  deep-seated  reactions  whereby 
minerals  are  built  up,  in  the  so-called  zone  of  cementation.  The 
components,  descending  in  solution,  are  precipitated  as  anamorphic 
minerals  in  the  cavities  of  buried  rocks,  or  else  older  components  are 
reorganized  into  new  minerals.  The  antithesis  is  katamorphic, 
which  refers  to  the  breaking  down  of  compounds  in  the  upper  zone  or 
zone  of  weathering,  or  oxidation.  The  katamorphic  reactions  largely 
provide  the  materials  for  anamorphic  reconstruction. 

Anatezis,  a  name  suggested  by  J.  J.  Sederholm  for  the  process  of  the 
recomposition  into  a  new  and  fresh  igneous  rock,  of  the  weathered  and 
disintegrated  components  of  an  older  one,  when  they  are  deeply  buried 
in  the  earth.  (Bull.  Com.  Geol.  de  Finlande,  23,  1907.) 

Andalusite-hornstone,  a  compact  contact  rock  containing  andalusite. 
It  is  usually  produced  from  shales  or  slates  by  intrusions  of  granite. 

Andendiorite,  a  tertiary,  quartz-augite-diorite,  which  occurs  in  areas 
like  islands  in  the  midst  of  the  volcanic  rocks  of  the  Chilean  Andes. 
The  quartzes  are  remarkable  for  their  inclusions  of  glass  and  of  fluid 
with  salt  crystals.  A.  W.  Stelzner,  Beitrage  Geol.  d.  Argent.  Republik, 
212,  1885. 

Andengranite,  a  biotite  bearing  hornblende-granite,  similar  in  occur- 
rence and  microscopic  features  to  the  andendiorite,  1.  c.,  208. 

Andesite,  volcanic  rocks  of  porphyritic  or  felsitic  texture,  whose 
crystallized  minerals  are  plagioclase  and  one  or  more  of  the  following: 
biotite,  hornblende  and  augite.  The  name  was  suggested  by  L.  von 
Buch  in  1836,  for  certain  rocks  from  the  Andes,  resembling  trachytes, 
but  whose  feldspar  was  at  first  thought  to  be  albite,  and  later  oligoclase. 
See  p.  62. 

Anhedron,  a  name  proposed  by  L.  V.  Pirsson  for  the  individual, 
mineral  components  of  igneous  rock,  that  lack  crystal  boundaries,  and 
that  cannot  therefore  be  properly  called  crystals  according  to  the  older 
and  most  generally  accepted  conception  of  a  crystal.  Xenomorphic 
and  allotriomorphic  are  adjectives  implying  the  same  conception.  The 
name  means  without  planes.  Bulletin  Geol.  Society  of  America,  Vol. 
VII.,  p.  492,  1895. 

Anhydrite;  the  name  of  the  mineral  is  also  used  as  the  name  of  a  rock, 
when  entire  beds  of  it  appear  in  a  sedimentary  series,  most  often  in 
association  with  rock-salt  and  gypsum.  Anhydrite  in  CaOSOs- 

Anogene,  a  general  name  for  rocks  that  have  come  up  from  below; 
i.  e.,  eruptive  rocks.  See  p.  15. 

Anorthite-rock,  a  name  given  by  R.  D.  Irving  to  a  coarsely  crystal- 


186  A  HAND  BOOK  OF  ROCKS. 

line,  granitoid  rock,  that  consists  almost  entirely  of  anorthite  (Mono- 
graph V.,  U.  S.  Geol.  Survey,  p.  59).  It  was  observed  on  the  Minne- 
sota shore  of  Lake  Superior.  The  rock  is  a  feldspathic  extreme  of  the 
gabbro  group,  practically  an  anorthosite  formed  of  anorthite. 

Anorthosite,  a  name  applied  by  T.  Sterry  Hunt  (Geol.  Survey- 
Canada,  1863,  22)  to  granitoid  rocks  that  consist  of  little  else  than 
labradorite  and  that  are  of  great  extent  in  eastern  Canada  and  the  Adi- 
rondacks.  The  name  is  derived  from  anorthose,  the  French  word  for 
plagioclase,  and  is  not  to  be  confounded  with  anorthite,  with  which  it  has 
no  necessary  connection,  although  anorthosite  is  used  as  a  general  name 
for  rocks  composed  of  plagioclase.  Mt.  Marcy  and  the  neighboring 
high  peaks  of  the  Adirondacks  are  formed  of  it.  The  rocks  are  extremes 
of  the  gabbro  group,  into  whose  typical  members  they  shade  by  insen- 
sible gradations.  See  p.  80. 

Apachite,  a  name  suggested  by  Osann,  from  the  Apache,  or  Davis 
Mountains  of  western  Texas,  for  a  variety  of  phonolite,  that  varies  from 
typical  phonolites  in  two  particulars.  It  has  almost  as  much  of  amphi- 
boles  and  of  aenigmatite  as  of  pyroxene,  whereas  in  normal  phonolites 
the  former  are  rare.  The  feldspars  of  the  groundmass  are  generally 
microperthitic.  Tscher.  Mitth.,  XV.,  454. 

Aphanite,  an  old  name,  now  practically  obsolete,  for  dense,  dark 
rocks,  whose  components  are  too  small  to  be  distinguished  with  the  eye. 
It  was  chiefly  applied  to  finely  crystalline  diabases.  An  adjective, 
aphanitic,  is  still  more  or  less  current. 

Aplite  is  now  chiefly  applied  to  the  muscovite-granite  that  occurs  in 
dikes,  and  that  is,  as  a  rule,  finely  crystalline.  Its  original  application 
was  to  granites  poor  or  lacking  in  mica.  See  p.  34.  The  name  is  from 
the  Greek  for  simple. 

Apo,  the  Greek  preposition  for  "from,"  suggested  by  F.  Bascom  as 
a  prefix  to  the  names  of  various  volcanic  rocks  to  describe  the  devitrified 
or  silicified  varieties,  mostly  of  ancient  date,  that  result  from  them,  and 
that  indicate  their  originals  only  by  the  preservation  of  characteristic 
textures.  Thus  apobsidian,  aporhyolite,  apandesite,  apobasalt,  etc., 
have  been  used.  See  p.  31.  Many  rocks  called  by  the  old  indefinite 
name  petrosilex  are  of  this  character.  Journal  of  Geology,  I.,  828, 
Dec.,  1893. 

Arenaceous,  an  adjective  applied  to  rocks  that  have  been  derived 
from  sand,  or  that  contain  sand. 

Arenite,  a  general  name,  suggested  by  A.  W.  Grabau  for  fragmental 
rocks  of  all  sorts,  whose  grain  is  that  of  sand.  Prefixes  are  used  to 
indicate  the  nature  of  the  sand,  as  silicarenite,  for  quartz  sands;  cal- 


GLOSSARY.  187 

carenite  for  limestone  composed  of  calcite  grains,  etc.  The  name  is 
especially  significant  as  applied  to  limestones.  (Bulletin  Geol.  Soc. 
Amer.  14:  348-352.) 

Argillite,  a  synonym  of  slate. 

Ariegite,  a  name  given  by  A.  Lacroix  to  a  special  family  of  grani- 
toid rocks,  consisting  primarily  of  monoclinic  pyroxene  and  spinel. 
Subvarieties  result  from  the  presence  of  amphibole  and  garnet.  The 
rocks  are  found  in  the  French  Pyrenees,  in  the  department  of  Ariege, 
from  which  they  take  their  name.  They  are  most  closely  related  to  the 
pyroxenites.  Comptes  Rendus,  VIII.,  International  Geological  Con- 
gress, 809,  1901. 

Arkite,  a  name  based  on  the  common  abbreviation  Ark.  for  Arkansas, 
and  given  by  H.  S.  Washington  to  a  rock  which  occurs  near  the  Diamond 
Jo  quarry,  Magnet  Cove,  Ark.  The  rock  was  earlier  called  leucite- 
porphyry,  by  J.  F.  Williams.  Washington  defines  it  as  '  a  holocrystal- 
line  porphyritic,  leucocratic  combination  of  leucite  (or  pseudoleucite) 
and  nephelite  with  pyroxene  and  garnet."  Jour.  Geology,  IX.,  615— 
617,  1901. 

Arkose,  a  special  name  for  a  sandstone  rich  in  feldspar  fragments,  as 
distinguished  from  the  more  common,  richly  quartzose  varieties.  See 
p.  98. 

Aschaffite,  a  name  suggested  by  Giimbel  for  a  dike  rock  occurring 
near  Aschaffenburg,  Bavaria.  (Bavaria,  Vol.  IV.,  Heft  II,  p.  23.)  It 
is  defined  by  Rosenbusch  as  a  dioritic,  dike  rock,  containing  quartz 
and  plagioclase,  with  biotite  as  the  chief  dark  silicate. 

Aschistic  applied  to  dikes,  which  represent  an  original  and  undif- 
ferentiated  or  undivided  magma — whereas,  complementary  or  diaschistic 
dikes  of  contrasted  acidic  and  basic  composition  represent  a  differenti- 
ation or  division  into  two.  Aschistic  literally  means  undivided  and 
diaschistic,  divided.  (Pirsson,  Rocks  and  Rock-Minerals,  169.) 

Ashbed  diabase,  a  local  name  used  on  Keweenaw  Point,  Lake 
Superior,  for  a  rock  resembling  a  conglomerate,  but  which  is  interpreted 
by  Wadsworth  as  a  very  scoriaceous,  amygdaloidal  sheet  into  which 
much  sand  was  washed  in  its  early  history.  See  Monograph  V.,  U.  S. 
Geol.  Surv.,  p.  138. 

Asiderite,  Daubree's  name  for  stony  meteorites  that  lack  metallic 
iron. 

Asperite,  a  collective  name  suggeted  by  G.  F.  Becker  for  the  rough 
cellular  lavas  whose  chief  feldspar  is  plagioclase,  but  of  which  it  is  im- 
possible to  speak  more  closely  without  microscopic  determination. 
The  name  is  intended  for  general  field  use  much  as  trachyte  was  em- 


i88  A  HAND  BOOK  OF  ROCKS. 

ployed  in  former  years.  It  is  coined  from  the  Latin  word  for  rough. 
Also  Monograph  XIII.,  U.  S.  Geol.  Surv.,  p.  151. 

Atazite.     See  under  Taxite. 

Atmogenic,  a  general  name  proposed  by  A.  W.  Grabau  for  the  rocks 
which  are  due  to  the  work  of  the  atmosphere,  i,  e.,  the  aeolian  rocks 
and  those  due  to  atmospheric  weathering.  It  forms  one  in  a  logical 
sequence,  pyrogenic;  hydrogenic;  atmogenic. 

Augen,  the  German  word  for  eyes;  used  as  a  prefix  before  various 
rock  names,  but  more  especially  gneiss,  to  describe  larger  minerals  or 
aggregates  of  minerals,  which  are  in  contrast  with  the  rest  of  the  rock. 
In  the  gneisses,  feldspars  commonly  form  the  augen  and  are  lenticular 
with  the  laminations  forking  around  them,  in  a  way  strongly  suggesting 
an  eye.  The  term  is  seldom  used  in  any  other  connection  than  with 
gneiss  in  America. 

Augite,  the  commonest  rock-making  pyroxene.  As  distinguished 
from  other  pyroxenes  augite  refers  to  the  dark  varieties  with  consider- 
able alumina  and  iron.  The  name  is  used  as  a  descriptive  prefix  to 
many  rocks  that  contain  the  mineral,  as  for  instance  augite-andesite, 
augite-diorite,  augite-gneiss,  augite-granite,  augite-syenite,  etc. 

Augitite,  non-feldspathic,  porphyritic  rocks  consisting  essentially  of 
a  glassy  groundmass,  with  disseminated  augite  and  magnetite.  Various 
minor  accessories  also  occur.  The  name  was  first  applied  by  Doelter 
to  lavas  from  the  Cape  Verde  Islands.  (Verhandl.  d.  k.  k.  Geol.  Reichs- 
anst.,  1882,  143.)  See  above,  pp.  73,  74. 

Augitophyre,  a  basalt  with  phenocrysts  of  augite. 

Aureole,  the  area  that  is  affected  by  contact  metamorphism  around 
an  igneous  intrusion.  See  p.  125. 

Authigenous,  an  adjective  coined  by  Kalkowsky  to  describe  those 
minerals  which  form  in  sediments  after  their  deposition,  as  for  instance 
during  metamorphism.  The  name  emphasizes  in  its  etymology  the 
local  origin  of  the  minerals  as  contrasted  with  that  of  the  other  com- 
ponents, the  latter  having  been  brought  from  a  distance. 

Autochthonous,  an  adjective  derived  from  two  Greek  words,  mean- 
ing indigenous.  It  is  applied  to  those  rocks  that  have  originated  in 
situ,  such  as  rock  salt,  stalagmitic  limestones,  peat,  etc.,  but  it  is  of 
rare  use. 

Autoclastic,  an  adjective  applied  to  fragmental  rocks,  which  owe  their 
fragmental  character  to  crushing  or  dynamic  metamorphism,  and  not 
to  sedimentation. 

Automorphic  is  the  contrasted  term  with  xenomorphic  or  allotrio- 
morphic,  and  is  used  to  describe  those  minerals  in  rocks,  which  have 


GLOSSARY.  189 

their  own  crystal  boundaries.  The  later  suggested  word,  idiomorphic, 
means  the  same  thing  and  is  somewhat  more  widely  used. 

Avezacite,  a  name  given  by  A.  Lacroix  to  a  peculiar,  cataclastic 
rock  now  found  in  veins  or  dikes  in  a  peridotite  at  Avezac-Prat,  in  the 
French  Pyrenees.  The  rock  is  dense,  black  and  brittle,  but  contains 
large  basaltic  hornblendes  and  yellow  sphenes,  in  a  fine-grained  mass, 
which,  on  microscopic  examination  is  resolved  into  a  cataclastic  aggre- 
gate of  apatite,  sphene,  titaniferous  magnetite,  ilmenite,  hornblende, 
augite,  and  rarely  olivine  and  biotite.  It  is  supposed  to  have  resulted 
from  the  crushing  of  basic  pegmatitic  veins  or  dikes.  Comptes  Rendus, 
VIII.,  International  Geological  Congress,  826-829,  1901. 

Axiolite,  a  term  coined  by  Zirkel  in  his  report  on  Microscopical 
Petrography,  for  the  U.  S.  Geol.  Survey  along  the  Fortieth  Parallel, 
1876,  to  describe  those  spherulitic  aggregates  that  are  grouped  around 
an  axis  rather  than  around  a  point.  The  application  comes  in  micro- 
scopic work  rather  than  in  ordinary  determination. 


Banakite,  a  general  name  given  by  Iddings  to  a  group  of  igneous 
rocks  in  the  eastern  portion  of  the  Yellowstone  Park,  and  chiefly  in 
dikes.  They  are  porphyritic  and  richly  feldspathic.  The  phenocrysts 
are  labradorite  and  the  groundmass  consists  of  alkali-feldspars.  A  little 
biotite  and  subordinate  augite  may  be  present.  Chemically  they  range 
SiO2,  51-61;  A12O3,  16.7-19.6;  CaO,  3.5-6;  MgO,  1-4;  Na20,  3-8-4-5: 
K2O,  4.4-5.7.  The  group  should  be  considered  in  connection  with 
absarokite  and  shoshonite.  Jour,  of  Geol.,  III.,  937. 

Banatite,  a  name  coined  by  B.  v.  Cotta  in  1865  to  describe  the 
dioritic  rocks  that  are  connected  with  a  series  of  ore  deposits  in  the 
Austrian  province  of  the  Banat.  Accurate  microscopical  study  has 
shown  them  to  be  of  such  varying  mineralogy  that  the  name  has  now 
slight  definite  significance.  The  rocks  are  largely  quartz-diorites. 
Erzlagerstatten  im  Banat  und  in  Serbien,  1865. 

Barolite,  Wadsworth's  name  for  rocks  composed  of  barite  or  celestite. 
Kept,  of  State  Geol.  Mich.,  1891-92,  p.  93 

Barysphere,  a  term  for  the  deep  interior  portions  of  the  earth,  pre- 
sumably composed  of  heavy  metals  or  minerals.  It  is  contrasted  with 
lithosphere,  the  outer  stony  shell. 

Basalt,  a  word  of  ancient  but  uncertain  etymology  as  stated  on  p.  72. 
It  is  employed  as  a  rock  name  in  its  restricted  sense  for  porphyritic  and 
felsitic  rocks  consisting  of  augite,  olivine  and  plagioclase  with  varying 
amounts  of  a  glassy  base  which  may  entirely  disappear.  Its  subdivision 


190  A  HAND  BOOK  OF  ROCKS. 

into  basalt  and  basalt-porphyry,  on  the  proportions  of  phenocrysts, 
is  described  in  the  text.  In  a  broader  sense  the  basalt  or  basaltic  group 
is  used  to  include  all  the  dark,  basic  volcanic  rocks,  such  as  the  true 
basalts;  the  nepheline-,  leucite-  and  melilite-basalts;  the  augites  and 
limburgites;  the  diabases,  and  melaphyres.  The  word  basalt  is  an 
extremely  useful  field  name,  as  in  many  instances  the  finer  discrimi- 
nations can  only  be  made  with  the  microscope. 

Basanite,  a  very  old  term,  first  used  as  a  synonym  of  basalt;  also 
formerly  applied  to  the  black,  finely  crystalline  quartzite,  used  by  old- 
time  workers  in  the  precious  metals  as  a  touch-stone  or  test-stone  to 
distinguish  gold  from  brass  by  the  streak.  This  variety  was  often  called 
Lydian  stone  or  lydite.  Basanite  is  now  universally  employed  for 
those  volcanic  rocks,  that  possess  a  porphyritic  or  felsitic  texture  and 
that  contain  plagioclase,  augite,  olivine  and  nepheline  or  leucite,  one 
or  both,  each  variety  being  distinguished  by  the  prefix  of  one  or  the 
other,  or  of  both  of  the  last  named  minerals.  See  p.  72. 

Basanitoid,  a  term  suggested  by  Bucking  for  basaltic  rocks,  without 
definite  nepheline,  but  with  a  gelatinizing,  glassy  base  (H.  Bucking, 
Jahrb.  d.  k.  k.  preus.  Landesanst.,  1882). 

Base  or  Basis  is  employed  to  describe  that  part  of  a  fused  rock 
magma  that  in  cooling  fails  to  crystallize  as  recognizable  minerals,  but 
chills  as  a  glass  or  related,  amorphous  aggregate.  It  differs  thus  from 
groundmass,  which  is  the  relatively  fine  portion  of  a  porphyritic  rock  as 
distinguished  from  the  phenocrysts. 

Basic,  a  general  descriptive  term  for  those  igneous  rocks  that  are 
comparatively  low  in  silica.  55  or  50  per  cent,  is  the  superior  limit. 
See  also  Acidic  and  Medium. 

Batholite,  a  name  suggested  by  Suess  for  the  vast  irregular  masses  of 
plutonic  rocks  that  have  crystallized  in  depth  and  that  have  only  been 
exposed  by  erosion.  See  p.  15.  The  word  is  also  spelled  bathylite, 
and  batholith.  The  last  named  is  now  generally  preferred. 

Bed,  the  smallest  division  of  a  stratified  series,  and  marked  by  a  more 
or  less  well-defined  divisional  plane  from  its  neighbors  above  and 
below. 

Beerbachite,  a  name  given  by  Chelius  to  certain  small  dikes,  asso- 
ciated with  and  pentrating  large,  gabbro  masses,  and  having  themselves 
the  composition  and  texture  of  gabbro.  The  name  was  coined  in  the 
attempt  to  carry  out  the  questionable  separation  of  the  dike  rocks  from 
large,  plutonic  or  volcanic  masses  of  the  same  mineralogy  and  often  prac- 
tically the  same  structures.  Notizbl.  Ver.  Erdkunde,  Darmstadt,  1892, 
Heft  13,  p.  i. 


GLOSSARY.  19 1 

Bekinkinite,  a  name  derived  from  the  Bekinka  mountain  in  Madagas- 
car and  coined  by  H.  Rosenbusch  (Massige  Gesteine,  4th  ed.,  p.  441) 
for  a  variety  of  ijolite  originally  described  by  A.  Lacroix.  The  rock 
consists  of  about  75  per  cent,  titanaugite,  with  nephelite,  as  the  other 
chief  constituent.  There  is  some  anorthoclase,  and  accessory  olivine, 
apatite  and  leucoxene.  Bekinkinite  is  believed  to  correspond  to  a  deep- 
seated  nephelite-basalt. 

Belonite,  rod  or  club-shaped  microscopic  minerals,  which  usually 
occur  as  embryonic  crystals  in  a  glassy  rock. 

Belugite,  a  name  based  upon  the  Beluga  river,  Alaska,  and  suggested 
by  J.  E.  Spurr  for  a  transition  group  of  plagioclase  rocks  between  his 
diorites  and  diabases.  Spurr  restricts  the  name  diorite  to  those  plagio- 
clase rocks  (without  regard  to  the  dark  silicate)  whose  plagioclase  be- 
longs in  the  andesine-oligoclase  series.  The  diabase  group  on  the  other 
hand  contains  those  whose  plagioclase  belongs  in  the  labradorite- 
anorthite  series.  Belugites  with  a  porphyritic  texture  and  a  fine- 
grained or  aphanitic  groundmass  are  called  aleutites.  Amer.  Geol., 
XXV.,  231,  1900.  20th  Ann.  Rep.  U.  S.  Geol.  Survey,  Part  7,  195. 

Benches,  a  name  applied  to  ledges  of  all  kinds  of  rock  that  are  shaped 
like  steps  or  terraces.  They  may  be  developed  either  naturally  in  the 
ordinary  processes  of  land-degradation,  faulting,  and  the  like;  or  by 
artificial  excavation  in  mines  and  quarries. 

Beresite,  a  name  coined  by  Rose  many  years  ago  for  a  muscovite- 
granite  that  forms  dikes  in  the  gold  district  of  Beresovsk  in  the  Urals. 
It  is,  therefore,  practically  a  synonym  of  aplite,  as  earlier  defined,  but 
some  of  the  beresites  have  since  been  shown  to  be  practically  without 
feldspar,  and  to  form  a  very  exceptional  aggregate  of  quartz  and  musco- 
vite.  (Arzruni,  Zeitsch.  d.  d.  g.,  Gesellsch.,  1885,  865.)  The  muscovite 
may  be  secondary  and  really  sericite. 

Binary-granite,  a  term  used  in  older  geological  writings  for  those 
varieties  of  granite  that  are  chiefly  quartz  and  feldspar.  See  p.  35.  It 
has  recently  been  applied  to  granites  with  two  micas.  C.  R.  Keyes,  15th 
Ann.  Rep.  Dir.  U.  S.  Geol.  Survey,  714. 

Biotite  is  used  as  a  prefix  to  many  names  of  rocks  that  contain  this 
mica;  such  as  biotite-andesite,  biotite-gneiss,  biotite-granite,  etc. 

Bituminous,  an  adjective  applied  to  rocks  with  much  organic,  or  at 
least  carbonaceous  matter,  mostly  in  the  form  of  the  hydrocarbons  which 
are  usually  described  as  bitumen. 

Blackband,  the  black,  bituminous  carbonate  of  iron,  which  forms 
beds  in  sedimentary  series  containing  coal-seams.  Although  once  an 
object  of  mining  in  Ohio,  West  Virginia  and  Pennsylvania,  it  has  prac- 
tically dropped  out  of  use  in  the  United  States. 


I92  A  HAND  BOOK  OF  ROCKS. 

Blairmorite,  an  analcite-trachyte  named  by  Cyril  Knight  after  Blair- 
more,  a  town  in  southwestern  Alberta,  Can.,  near  the  Crows  Nest  Pass 
coal  fields.  Although  only  found  as  yet  in  tuffs,  the  rock  is  recognizable 
as  a  new  species.  Phenocrysts  of  orthoclase  and  analcite  (at  first  con- 
sidered leucite)  are  set  in  a  groundmass  of  predominant  orthoclase  rods, 
some  plagioclase,  analcite  and  titanite.  (Cyril  Knight,  Canadian 
Record  of  Science,  IX.,  275,  1905.) 

Blavierite,  a  peculiar  contact  rock  occurring  at  several  places  in  the 
ancient  massifs  of  Mayenne  and  of  the  Pyrenees,  in  France,  It  results 
from  the  action  of  intrusive  microgranites,  upon  sericite  schists.  While 
preserving  the  schistose  structure,  it  has  in  addition  to  the  fine  micaceous 
components  of  the  schist,  dihexahedral  quartzes  with  orthoclase  and 
oligoclase,  apparently  referable  to  the  intrusive.  (L.  Bergeron,  Bull. 
Soc.  geol.  de  France,  1888  (3),  XVII.,  58.) 

Blue-ground,  local  miners'  name  for  the  decomposed  peridotite  or 
kimberlite  that  carries  the  diamonds  in  the  South  African  mines. 

Bojite,  a  name  given  by  E.  Weinschenk  to  a  variety  of  gabbro,  which 
occurs  in  association  with  the  graphite  of  northern  Bavaria.  It  differs 
from  normal  gabbro  in  containing  hornblende,  in  addition  to  augite, 
and  the  name  is  intended  to  indicate  a  group  of  hornblende  gabbros, 
just  as  norite  implies  those  with  hypersthene.  The  original  bojite  con- 
tained brown  hornblende,  colorless  pyroxene,  and  reddish  brown  biotite. 
In  amount  equal  to  all  of  these  is  a  plagioclase  (labradorite-bytownite), 
often  not  multiple-twinned.  As  accessories  there  were  zircon,  apatite, 
titanite,  pyrite  and  magnetite.  Abh.  d.  k.  bay.  Akad.  d.  Wissensch., 
II  Classe,  XIX.,  33. 

Bombs,  masses  of  lava  expelled  from  a  volcano  by  explosions  of 
steam.  They  fall  as  rounded  masses  and  lie  on  the  slopes  of  the  cone, 
or  become  buried  in  tuffs. 

Boninite,  Petersen's  name  for  a  glassy  phase  of  andesite  with  bron- 
zite,  augite  and  a  little  olivine,  from  the  Bonin  Islands,  Japan.  Jahrb. 
Hamburg  Wissensch.  Anst.,  VIII.,  1891.  Compare  sanukite. 

Borolanite,  a  rare  rock  related  to  the  nephelite-syenites  and  described 
by  Home  and  Teall  from  Borolan,  Sutherlandshire,  Scotland.  It 
has  granitoid  texture,  and  consists  principally  of  orthoclase  and  the 
variety  of  garnet  called  melanite.  As  accessory  minerals,  biotite,  py- 
roxene, alteration  products  of  nephelite,  sodalite,  titanite,  apatite 
and  magnetite  appear.  (Trans.  Roy.  Soc.  of  Edinburgh,  1892,  p.  163.) 

Bostonite,  a  name  proposed  by  Hunter  and  Rosenbusch  for  certain 
dikes,  having  practically  the  mineralogical  and  chemical  composition  of 
trachytes  or  porphyries,  except  that  anorthoclase  (and  therefore  soda) 


GLOSSARY.  193 

is  abnormally  abundant  and  dark  silicates  are  few  or  lacking  They 
are  much  the  same  as  dike-keratophyres  and  were  named  in  carrying 
out  the  questionable  separation  of  the  dike-rocks,  as  a  distinct  grand 
division  from  the  plutonic  and  volcanic  rocks.  The  name  was  suggested 
by  their  supposed  presence  near  Boston,  Mass.,  but  Marblehead,  20 
miles  or  more  distant,  is  their  nearest  locality.  They  have  been  since 
met  in  largest  amount  on  the  shores  of  Lake  Champlain  and  in  the  neigh- 
boring parts  of  Canada.  Tscher.  Min.  u.  Petrog.  Mitth.,  1890,  447. 
See  also  Bull.  107,  U.  S.  Geol.  Survey. 

Bouteillenstein,  i.  e.,  bottlestone,  a  peculiar  green  and  very  pure 
glass,  found  as  rolled  pebbles  near  Moldau,  Bohemia.  It  is  also  called 
moldauite  and  pseudochrysolite,  the  latter  from  its  resemblance  to 
olivine.  It  is  not  certainly  a  terrestrial  rock.  In  later  years  many 
have  believed  the  pebbles  to  be  small  meteorites  of  unusual  compo- 
sition. 

Boulder-clay,  unsorted  glacial  deposits,  consisting  of  boulders  and 
clay;  till;  hardpan. 

Breccia,  a  fragmental  rock  whose  components  are  angular  and  there- 
fore, as  distinguished  from  conglomerates,  are  not  water-worn.  There 
are  friction  or  fault  breccias,  talus-breccias  and  eruptive  breccias.  The 
word  is  of  Italian  origin.  See  p.  93. 

Broccatello,  an  Italian  word  for  a  brecciated  and  variegated  marble. 

Bronzite  is  often  used  as  a  prefix  to  the  names  of  rocks  containing 
the  mineral.  Rocks  of  the  gabbro  family  are  the  commonest  ones  that 
have  the  prefix. 

Brotocrystals,  etc.  A.  C.  Lane  has  suggested  the  following  five 
varieties  of  phenocrysts:  (i)  Brotocrystals,  those  phenocrysts  whose 
corroded  or  embayed  outlines  prove  that  they  were  formed  at  a  period 
antedating  the  eruptive  stage.  The  name  is  from  the  Greek  and  means 
''  eaten  "  or  ''  gnawed  "  crystal.  (2)  Rhyocrystal,  those  phenocrysts 
which  have  sharp,  crystallographic  outlines,  and  which  are  arranged 
in  flow-lines,  so  that  they  are  clearly  products  of  the  effusive  period. 
The  name,  suggested  by  F.  E.  Wright,  means  a  "  flow-crystal."  (3) 
Eocrystals,  those  phenocrysts  which  are  developed  under  the  conditions 
prevailing  in  the  magma  after  it  has  come  to  rest.  They  increase  uni- 
formly in  size  from  border  of  the  igneous  mass  or  most  rapidly  chilled 
portion,  to  the  center  or  most  slowly  cooled  portion.  The  name  means 
"  early  crystal."  (4)  Oriocrystals,  those  phenocrysts  whose  conditions 
of  formation  lie  midway  between  the  great  heat  of  the  original  intrusive 
and  the  relative  coldness  of  the  walls.  As  the  former  cools  and  the 
latter  heats  up,  conditions  are  established  at  the  border  which  very 
13 


194  A  HAND  BOOK  OF  ROCKS. 

slowly  change  and  which  permit  the  growth  of  large  individuals.  The 
word  means  "  border  crystal."  (5)  Metacrystals,  relatively  large  crystals 
in  metamorphosed  sedimentary  and  igneous  rocks,  such  as  staurolite, 
garnet,  andalusite,  etc.  The  word  means  metamorphic  crystal.  (Bull. 
Geol.  Soc.  Amer.,  XIV.,  386-388,  1902.) 

Buchite,  sandstones  whose  argillaceous  or  calcareous  cement  has 
been  melted  with  more  or  less  of  the  quartz  grains  into  a  glass  by  the 
contact  influence  of  neighboring  basalt.  (Zirkel,  Lehrbuch  der  Pe- 
trographie  3,  99,  1894.) 

Buchnerite,  a  name  proposed  by  Wadsworth  for  those  peridotites, 
terrestrial  and  meteoritic,  which  consist  of  olivine,  enstatite  (bronzite) 
and  augite.  The  name  was  given  in  honor  of  Dr.  Otto  Buchner,  an 
authority  on  meteorites.  Lithological  Studies,  1884,  p.  85. 

Buchonite,  a  special  name  given  by  Sandberger  to  a  nephelite-teph- 
rite  that  contains  hornblende.  Sitzungsberichte  d.  Berl.  Akad.  Wiss., 
July,  1872,  203;  1873,  vi. 

Buhrstone,  asilicified,  fossiliferous  limestone,  with  abundant  cavities 
which  were  formerly  occupied  by  fossil  shells.  Its  cellular  character 
and  toughness  occasioned  its  extensive  use  as  a  millstone  in  former  years. 

Bysmalith,  a  name  suggested  by  J.  P.  Iddings  for  an  igneous  intru- 
sion that  forms  a  huge  cylindrical  mass  or  plug,  with  length  and  width 
approximately  the  same,  but  of  relatively  great  height.  Journal  of 
Geology,  VI.,  704. 

c 

Calcarenite,  a  name  suggested  by  A.  W.  Grabau  for  a  "  limestone  or 
dolomite  composed  of  coral  or  shell  sand  or  of  lime  sand  derived  from 
the  erosion  of  older  limestones."  The  name  is  from  the  Latin  for  lime 
and  sand.  Bull.  Geol.  Soc.  Amer.,  XIV.,  349,  1903. 

Calcilutite,  a  name  suggested  by  A.  W.  Grabau  for  a  limestone  or 
dolomite  made  up  of  calcareous  rock  flour,  the  composition  of  which  is 
typically  non-siliceous,  though  many  calcilutites  have  an  intermixture  of 
clayey  material.  The  word  is  from  the  Latin  for  lime  and  mud.  Bull. 
Geol.  Soc.  Amer.,  XIV.,  350,  1903. 

Calcirudite,  a  name  suggested  by  A.  W.  Grabau  for  a  "  limestone  or 
dolomite  composed  of  broken  or  worn  fragments  of  coral  or  shells  or  of 
limestone  fragments,  the  interstices  filled  with  lime,  sand  or  mud,  and 
with  a  lime  cement."  The  word  is  derived  from  the  Latin  for  lime  and 
rubble.  Bull.  Geol.  Soc.  Amer.,  XIV.,  349,  1903. 

Calc-schist,  schistose  rocks,  rich  in  calcite  or  dolomite  and  forming 
intermediate  or  transitional  rocks  between  the  mica-schists  and  crystal- 
line limestones.  See  p.  129.  Compare  cipolin. 


GLOSSARY.  195 

Camptonite,  a  name  given  by  Rosenbusch  to  certain  dike  rocks, 
having  in  typical  cases  the  mineralogical  composition  of  diorites,  i.  e.t 
with  dark  brown  hornblende,  plagioclase,  magnetite,  and  more  or  less 
augite.  They  are  often  porphyritic  in  texture,  and  may  even  have  a 
glassy  groundmass.  Without  the  microscope  camptonites  usually 
appear  as  dark  basaltic  rocks  with  a  few  shining  crystals  of  hornblende  or 
augite;  their  determination  is  essentially  microscopic.  Intimately  asso- 
ciated with  the  camptonites  of  typical  composition  have  been  found 
others  corresponding  to  all  varieties  of  basaltic  rocks.  Such  with  pre- 
vailing augite  have  been  called  augite-camptonite.  The  name  campto- 
nite  is  derived  from  the  township  of  Campton,  in  the  Pemigewasset 
Valley,  N.  H.  The  original  camptonites  were  discovered  near  Liver- 
more  Falls,  on  the  Pemigewasset  river,  many  years  ago,  by  O.  P. 
Hubbard.  They  were  microscopically  described  by  G.  W.  Hawes  in 
1878,  and  on  this  determination  Rosenbusch  based  the  name.  They, 
or  their  near  relatives,  have  often  intimate  associations  with  nephelite 
syenites.  (See  also,  monchiquite,  fourchite,  ouachitite.)  Camptonites 
are  especially  abundant  throughout  the  Green  Mountains  and  near 
Montreal.  G.  W.  Hawes,  Amer.  Jour.  Sci.,  1879,  XVII.,  147.  H. 
Rosenbusch,  Massige  Gesteine,  1887,  333.  Bulletin  107,  U.  S.  Geol. 
Surv. 

Cancrinite,  the  name  of  the  mineral  is  sometimes  prefixed  to  the 
names  of  rocks  containing  it,  as  cancrinite-syenite. 

Carbonolite,  Wadsworth's  name  for  carbonaceous  rocks.  Rept.  State 
Geol.  Mich.,  1891-92,  p.  93. 

Canneloite,  a  name  given  by  A.  C.  Lawson  to  a  group  of  eruptive 
rocks  at  Carmelo  Bay,  Calif.,  which  are  intermediate  between  the 
basalts  and  andesites.  They  range  in  silica  from  52  to  60  per  cent., 
have  augite  and  plagioclase  for  phenocrysts;  and  a  peculiar,  orthorhom- 
bic,  hydrated  silicate  of  iron,  lime,  magnesia  and  soda,  which  is  a  sec- 
ondary mineral  after  some  original,  probably  olivine.  The  secondary 
mineral  has  been  called  Iddingsite.  Bull.  Geol.  Dept.  Univ.  of  Calif., 
I.,  29,  1893. 

Cascadite,  an  adaptation  by  Rosenbusch  into  the  forms  of  the  custom- 
ary nomenclature  of  Pirsson's  term  cascadose,  of  the  quantitative  sys- 
tem. The  name  is  derived  from  Cascade  Co.,  Mont.,  where  the  rock, 
which  is  a  variety  of  minette,  occurs.  (Bull.  U.  S.  Geol.  Survey,  237, 
p.  149.  Rosenbusch,  Mikros.  Phys.,  4th  ed.,  II.,  698.) 

Cataclastic,  a  structural  term  applied  to  those  rocks  that  have  suffered 
mechanical  crushing  in  dynamic  metamorphism.  Compare  autoclastic. 

Catawberite,  a  name  given  bv  O.  Lieber  to  a  rock  in  South  Caro- 


196  A  HAND  BOOK  OF  ROCKS. 

lina  that  is  an  intimate  mixture  of  talc  and  magnetite.     Gangstudien, 

III.,  353,  359- 

Catlinite,  a  local  name  in  Minnesota  for  a  red,  siliceous  argillite,  pre- 
sumably of  Cambrian  age,  that  was  used  by  the  Indians  for  pipe  bowls. 
C.  T.  Jackson,  Amer.  Jour.  Sci.,  1839,  388. 

Catogene,  i.  e.,  sedimentary  rocks,  whose  particles  have  sunk  from 
above  downward. 

Cement,  the  material  that  binds  together  the  particles  of  a  fragmental 
rock.  It  is  usually  calcareous,  siliceous  or  ferruginous.  See  p.  96. 
The  word  is  also  used  in  gold-mining  regions  to  describe  various  con- 
solidated, fragmental  aggregates,  such  as  breccia,  conglomerate  and  the 
like,  that  are  auriferous. 

Chalk,  a  marine,  calcareous  and  excessively  fine,  organic  sediment 
usually  consolidated. 

Charnockile,  a  name  given  by  T.  H.  Holland  to  an  ancient  series  of 
hypersthenic  gneisses  in  India  and  only  intended  for  local  use.  Memoirs 
Geol.  Survey  India,  XXVIII.,  130,  1900. 

Cherokite,  a  name  derived  from  the  Cherokee,  Mississippian  lime- 
stone of  southwest  Missouri  and  coined  by  W.  P.  Jenney  to  describe 
the  dense,  hard,  brown  or  drab,  silicified,  residual  sands,  which  con- 
stitute the  cement  of  the  chert  breccias  in  the  zinc  mines  of  the  Joplin 
district.  (Trans.  Amer.  Inst.  Min.  Eng.,  22:  195,  1894.) 

Chert,  a  compact,  siliceous  rock  formed  of  chalcedonic  or  opaline 
silica,  one  or  both,  and  of  organic  or  precipitated  origin.  See  pp. 
116,  117.  Cherts  often  occur  distributed  through  limestone,  affording 
cherty  limestones.  Flint  is  a  variety  of  chert.  Cherts  are  especially 
common  in  the  Lower  Carboniferous  rocks  of  southwest  Missouri. 

Chibinite,  a  name  derived  from  the  Russian  designation  of  the 
Finnish  locality,  better  known  among  petrographers  as  Umptek,  and 
applied  by  W.  Ramsay  to  a  variety  of  nephelite-syenite.  It  is  a  coarsely 
crystalline,  aggregate  of  microperthite,  nephelite,  aegirite,  titanite,  eudi- 
alyte,  lamprophyllite  and  rarer  minerals.  (Rosenbusch,  Mikros. 
Physiographie,  4th  ed.,  II.,  231.) 

Chlorite,  a  general  name  for  the  green,  secondary,  hydrated  silicates, 
which  contain  alumina  and  iron,  and  which  are  especially  derived  from 
augite,  hornblende  and  biotite.  Chlorite  is  used  as  a  prefix  for  various 
names  of  rocks  that  contain  the  mineral,  such  as  chlorite  schist.  The 
name  is  coined  from  the  Greek  word  for  green. 

Chlorophyr,  a  name  given  by  A.  Dumont  to  certain  porphyritic 
quartz  diorites  near  Quenast,  Belgium.  See  Delesse,  Bull.  Soc.  Geol. 
de  France,  1850,  315. 


GLOSSARY.  197 

Chondri,  the  rounded  and  ellipsoidal  grains  of  silicates  which  are 
characteristic  of  meteorites.  In  section  they  suggest  grains  of  wheat 
or  barley,  packed  together,  a  fact  which  suggested  the  name  from  the 
Greek. 

Chonolith  (or  chonolite),  a  name  for  an  irregularly  shaped  mass  of 
intrusive  rock,  coined  by  R.  A.  Daly  from  the  Greek  words  for  mold 
(in  which  metal  is  cast)  and  rock.  "An  igneous  body  (a)  injected 
into  dislocated  rock  of  any  kinds  stratified  or  not;  (&)  of  shape  and 
relations  irregular  in  the  sense  that  they  are  not  those  of  a  true  dike, 
vein,  sheet,  laccolith,  bysmalith  or  neck;  and  (c)  composed  of  magma 
either  passively  squeezed  into  a  subterranean,  orogenic  chamber  or 
actively  forcing  apart  the  country-rocks.  Jour,  of  Geology,  XIII., 
499,  1905. 

Ciminite,  a  name  derived  from  the  Monti  Cimini  in  Italy,  and  given 
by  H.  S.  Washington  to  a  group  of  lavas,  intermediate  between  trachytes 
and  basalts.  They  are  porphyritic  in  texture  and  are  characterized  by 
the  presence  of  alkali  feldspar  and  basic  plagioclase,  augite  and  olivine, 
with  accessory  magnetite  and  apatite.  Biotite  and  hornblende  are 
either  absent  or  are  insignificant.  They  range  from  54  to  57  SiOz,  5-9 
CaO,  and  3-6  MgO.  Journal  of  Geology,  V.,  351,  1897.  Compare 
Latite. 

Cipolin,  a  marble  charged  with  mica,  usually  of  the  variety  phlogopite 
and  forming  a  transition  between  marbles  and  mica-schists,  two  rocks 
which  are  often  associated. 

Clastic,  a  descriptive  term  applied  to  rocks  formed  from  the  fragments 
of  other  rocks;  fragmental. 

Clay,  a  general  name  for  the  fine,  aluminous  sediments  that  are  plastic. 
Though  usually  soft,  they  may  be  so  hard  as  to  need  grinding  before  the 
plasticity  manifests  itself,  as  in  numerous  fire  clays.  See  p.  101. 

Clay-ironstone,  the  argillaceous  variety  of  spathic  iron  ore  which 
forms  concretions  and  beds  in  the  shales  of  coal  measures. 

Clay  slate,  metamorphosed  clay,  with  new  cleavages  developed  by 
pressure  and  shearing.  The  term  is  used  in  distinction  to  mica-slate, 
and  other  slaty  rocks.  See  p.  146. 

Claystone-porphyry,  an  old  and  somewhat  indefinite  name  for  those 
porphyries  whose  naturally  fine  groundmass  is  more  or  less  kaolinized 
so  as  to  be  soft  and  earthy,  suggesting  hardened  clay 

Clinkstone.     See  Phonolite. 

Comagmatic,  a  region  or  petrographic  province  whose  igneous  rocks 
possess  some  marked,  common  characteristic  such  as  relatively  high 
percentages  of  soda  or  potash  or  lime. 


198  A  HAND  BOOK  OF  ROCKS. 

Comendite,  a  variety  of  rhyolite,  containing  phenocrysts  of  sanidine, 
quartz  and  segirite,  in  a  granophyric  and  spherulitic  groundmass.  Horn- 
blende and  some  blue,  soda-amphibole,  together  with  zircon,  magnetite, 
titanite,  tridymite,  and  plagioclase  occur  as  accessories.  The  name 
was  given  by  Bertolis,  an  Italian  geologist,  from  a  locality  on  the  island 
of  San  Pietro,  Sardinia.  Rend.  Roy.  Acad.  Lincei,  IV.,  48,  1895. 
Compare  Paisanite. 

Complementary  Rocks,  a  term  suggested  by  W.  C.  Brogger  for  the 
basic  rocks,  which,  usually  in  the  form  of  dikes,  accompany  larger  in- 
trusions of  more  acidic  types,  and  "complement"  them  in  a  chemical 
sense.  Quar.  Jour.  Geol.  Soc.,  L.,  15,  1894.  Compare  Lampro- 
phyre,  Oxyphyre  and  Radial  Dikes. 

Composite  dike,  a  dike  formed  by  two  intrusions  of  different  ages 
which  have  entered  the  same  fissure  (W.  Judd,  Quar.  Jour.  Geol.  Soc., 
1893,  536). 

Concretions,  spheroidal  or  discoid  aggregates  formed  by  the  segre- 
gation and  precipitation  of  some  soluble  mineral  like  quartz  or  calcite 
around  a  nucleus,  which  is  often  a  fossil. 

Cone-in-cone,  a  curious  structure,  occasionally  met  in  clay  rocks, 
whereby  two  opposing  and  interlocking  sets  of  cones  or  pyramids  are 
developed,  with  their  axes  parallel  and  their  bases  in  approximately 
parallel  surfaces. 

Conglomerate,  consolidated  gravel.     See  p.  96. 

Consanguinity,  a  term  used  by  Iddings  to  describe  the  genetic  rela- 
tionship of  those  igneous  rocks  which  are  presumably  derived  from  a 
common,  parent  magma.  See  p.  91,  and  Bull.  Phil.  Soc.  Washington, 
XL,  89. 

Consertal,  a  texture  in  an  igneous  rock,  composed  of  irregularly 
shaped  crystals,  closely  fitted  together.  (Jour.  Geol.,  14:  700,  1906.) 

Contact,  the  place  or  surface  where  two  different  kinds  of  rocks  come 
together.  Although  used  for  sedimentary  rocks,  as  the  contact  between 
a  limestone  and  sandstone,  it  is  yet  more  characteristically  employed 
for  the  border  between  igneous  intrusions  and  their  walls.  The  word 
is  of  wide  use  in  western  mining  regions  on  account  of  the  frequent  oc- 
currence of  ore-bodies  along  contacts.  On  contact-metamorphism, 
see  pp.  125-130. 

Coquina,  a  fragmental  limestone  consisting  of  bits  of  shells,  cemented 
together  in  a  loosely  textured  aggregate.  The  rock  is  common  in 
Florida. 

Cordierite,  a  synonym  of  iolite  or  dichroite,  employed  as  a  prefix  to 
those  rocks  that  contain  the  mineral,  as  cordierite-gneiss. 


GLOSSARY.  199 

Cornubianite,  a  name  coined  by  Blase  from  the  classic  name  for 
Cornwall,  England,  to  describe  a  contact  hornfels,  consisting  of  andalu- 
site,  mica  and  quartz.  It  was  proposed  as  a  substitute  for  an  earlier 
but  indefinite  term  proteolite.  Bonney  suggests  restricting  cornubianite 
to  tourmaline  hornfels.  Quar.  Jour.  Geol.  Soc.,  1886,  104. 

Corona-structure.     See  Reaction-rims. 

Corrasion,  geological  term  for  the  wearing  away  of  rocks  by  grit  sus- 
pended in  moving  water  or  air:  to  be  distinguished  from  erosion. 

Corroded  Crystals,  phenocrysts  that  after  crystallization  are  more  or 
less  reabsorbed  or  fused  again  into  the  ma  ma. 

Corsite,  a  name  applied  by  Zirkel  to  the  orbicular  or  spheroidal 
diorite  from  Corsica:  synonym  of  napoleonite.  Lehrb.  d.  Petrographie, 
1896,  II.,  133,  320. 

Cortlandtite,  a  special  name  given  by  G.  H.  Williams  to  a  peridotite 
that  consists  chiefly  of  hornblende  and  olivine  and  that  occurs  in  the 
so-called  Cortlandt  series  of  igneous  rocks  in  the  township  of  Cortlandt, 
just  south  of  Peekskill,  on  the  Hudson  River.  This  rock  had  been 
previously  called  hudsonite  by  E.  Cohen,  a  name  rejected  by  Williams 
because  already  used  for  a  variety  of  pyroxene.  Amer.  Jour.  Sci.,  Jan., 
1886,  30. 

Corundolite,  Wadsworth's  name  for  rocks  composed  of  corundum  or 
emery.  Rept.  State  Geol.  Mich.,  1891-92,  p.  92. 

Corundum,  the  name  of  the  mineral  is  sometimes  prefixed  to  the 
names  of  rocks  containing  it;  as  corundum-syenite. 

Covite,  a  name  derived  from  Magnet  Cove,  Ark.,  and  suggested  by 
H.  S.  Washington  for  a  leucocratic,  holocrystalline  combination  of 
orthoclase  (alkali-feldspar)  and  less  nephelite,  with  hornblende  and 
aegirite-augite,  and  of  granitic  texture.  A  typical  analysis  is  given  in 
the  reference.  Jour.  Geol.,  IX.,  612-615,  1901.  The  rock  had  been 
previously  described  as  a  fine-grained  syenite,"  by  J.  F.  Williams. 

Crenitic,  a  word  derived  from  the  Greek  for  spring,  and  especially 
used  by  T.  S.  Hunt  for  those  rocks,  which  were  thought  by  him  to  have 
come  to  the  surface  in  solution  and  to  have  been  precipitated.  He  used 
the  so-called  "  crenitic  hypothesis"  to  explain  certain  schists  whose 
feldspars  were  supposed  to  have  been  originally  zeolites,  but  his  views 
have  received  slight,  if  any,  support.  Proc.  Roy.  Soc.  Canada,  Vol. 
II.,  Sec.  III.,  1884.  Reprint,  p.  25.  Crenitic  is  also  used  by  W.  O. 
Crosby  to  describe  those  mineral  veins  which  have  been  deposited  by 
uprising  springs.  Technology  Quarterly,  April,  1894,  p.  39. 

Cross-bedding,  or  Cross-stratification,  descriptive  terms  applied  to 
those  minor  or  subordinate  layers  in  sediments  that  are  limited  to  single 


200  A  HAND  BOOK  OF  ROCKS. 

beds,  but  that  are  inclined  to  the  general  stratification.  They  are 
caused  by  swift,  local  currents,  deltas,  or  swirling  wind-gusts,  and  are 
especially  characteristic  of  sandstones,  both  aqueous  and  eolian.  See 

PP.  94,  95- 

Gratification,  the  English  equivalent  of  a  term  suggested  by  Posepny 
for  those  deposits  of  minerals  and  ores  that  are  in  layers  or  crusts  and 
that,  therefore,  have  been  distinctively  deposited  from  solution.  Trans. 
Amer.  Inst.  Min.  Eng.,  XXIII.,  207,  1893. 

Crypto-crystalline,  formed  of  crystals  of  unresolvable  fineness,  but 
not  glassy.  A  submicrosopical  crystalline  aggregate. 

Crystallites,  the  term  is  most  properly  applied  only  to  small,  rudi- 
mentary or  embryonic  crystals,  not  referable  to  a  definite  species,  but  it 
is  also  used  as  a  general  term  for  microscopic  crystals. 

Cumberlandite,  a  name  derived  from  Iron  Mine  Hill  in  the  town  of 
Cumberland,  R.  I.,  proposed  by  Wadsworth  for  the  ultra-basic,  igne- 
ous rocks,  forming  the  hill.  It  is  an  aggregate  of  titaniferous  magne- 
tite, plagioclase,  olivine  and  secondary  minerals,  but  contains  from 
40-45  per  cent,  iron  oxides  and  about  10  per  cent.  TiO2.  Lithological 
Studies,  1884. 

Cumulites,  Vogelsang's  name  for  spherulitic  aggregates  of  globulites. 
Die  Krystalliten,  1875. 

Cumulophyric,  a  porphyritic  texture  in  which  the  phenocrysts  are 
in  irregular  groups.  (Jour.  Geol.  14:  703,  1906.) 

Cuselite,  Rosenbusch's  name  for  a  peculiar  variety  of  augite-porphy- 
rite  from  Cusel,  in  the  Saar  basin.  Massige  Gest.,  503,  1887. 

D 

Dacite,  quartz-bearing  andesites.  The  name  was  suggested  by  the 
ancient  Roman  province  of  Dacia,  now  in  modern  Hungary  See  p.  56. 

Dahamite,  a  name  derived  from  Dahamis,  a  place  on  the  island  of 
Socotra,  and  given  by  A.  Pelikan  to  a  dike  rock  of  brown  color,  con- 
pact  texture  with  red  phenocrysts  of  tabular  albite  or  albite-oligoclase. 
The  mineralogical  composition  as  shown  by  recasting  an  analysis  is 
albite,  43.8;  anorthite,  2.8;  orthoclase,  12.2;  quartz,  31.5;  riebeckite, 
6.8.  The  rock  appears  to  be  related  to  paisanite.  Denkschr.  d.  mat. 
natur.  wiss.  Classe  d.  k.  Akad.  d.  Wiss.  Vienna,  LXXI.,  1902.  Amer. 
Jour.  Sci.,  Nov.,  1902,  397. 

Damourite-schist,  a  micaceous  schist  whose  micaceous  mineral  is 
damourite.  Much  the  same  as  hydro-mica  schist.  See  p.  139. 

Dedolomitization,  the  alteration  of  a  dolomite  into  some  other  rock, 
as  into  a  serpentine. 


GLOSSARY.  201 

Dellenite,  a  name  proposed  by  Brogger  for  an  intermediate  group  of 
effusive  rocks,  between  the  dacites  and  the  liparites  (rhyolites).  The 
name  is  derived  from  Dellen,  Helsingland,  Sweden.  Die  Triadische 
Eruptionsfolge  bei  Predazzo,  p.  60,  and  footnote  to  p.  59.  Compare 
Toscanite. 

Desmosite,  a  banded  contact  rock  developed  from  shales  and  slates 
by  intrusions  of  diabase.  Compare  spilosite  and  adinole.  See  Zincken, 
Karsten  und  v.  Dechen's  Archiv,  XIX.,  584,  1845. 

Detritus,  a  general  name  for  incoherent  sediments,  produced  by  the 
wear  and  tear  of  rocks  through  the  various  geological  agencies.  The 
name  is  from  the  Latin  for  "  worn." 

Devitrification,  the  process  by  which  glassy  rocks  break  up  into  defi- 
nite minerals.  The  latter  are  usually  excessively  minute  but  are  chiefly 
quartz  and  feldspars. 

Diabase,  igneous  rocks,  in  sheets  or  dikes,  consisting  essentially  of 
plagioclase,  augite  and  magnetite,  with  or  without  olivine,  and  possess- 
ing a  texture  often  called  ophitic,  but  which  may,  perhaps,  be  better 
described  as  diabasic.  The  feldspars  are  lath-shaped  and  automorphic 
while  the  augite  is  xenomorphic  and  packed  in  their  interstices.  See 
p.  77,  and  also  Ophitic.  Diabase  has  had  a  somewhat  variable  sig- 
nificance during  its  history,  but  with  the  final  exit  of  the  time-element 
in  the  classification  of  igneous  rocks  its  present  meaning  is  generally 
accepted  as  above  given.  In  a  suggested  system  for  the  classification  of 
the  igneous  rocks  J.  E.  Spurr  has  used  it  as  a  group  name  for  those 
igneous  rocks  whose  chief  feldspar  is  a  plagioclase  belonging  to  the  labra- 
dorite-anorthite  series.  Subdivisions  are  then  based  on  textures. 
2Oth  Ann.  Rep.  U.  S.  Geol.  Surv.,  Part  7,  pp.  211-216.  Nevertheless  the 
statement  above  remains  correct.  Diabase  is  often  used  as  a  prefix 
for  double  names,  as  diabase-aphanite,  diabase-gabbro,  etc. 

Diabase-porphyrite,  a  porphyrite  whose  groundmass  is  a  finely  crys- 
talline diabase,  and  whose  phenocrysts  are  prevailingly  plagioclase.  It 
is  contrasted  with  augite-porphyrite,  whose  phenocrysts  are  prevailingly 
augite. 

Diagenesis,  the  solidification  of  clastic  rocks  by  static  metamorphism 
not  involving  much  if  any  recrystallization.  The  change  of  muds  to 
shales,  and  of  clays  to  slates  are  illustrations.  The  term  has  been 
especially  brought  forward  in  later  years  by  Joh.  Walther. 

Diallage,  the  variety  of  monoclinic  pyroxene  which  in  addition  to  the 
prismatic  cleavages,  has  others  parallel  to  the  vertical  pinacoids.  Used 
also  as  a  prefix  to  many  rocks  containing  the  mineral. 

Diaschistic.     See  Aschistic. 


202  A  HAND  BOOK  OF  ROCKS. 

Diatomaceous  earth,  rocks  essentially  formed  of  the  abandoned  frus- 
tules  of  the  microscopic  organisms  called  diatoms. 

Dichroite,  see  under  cordierite. 

Dikes,  spelled  also  dykes,  intrusions  of  igneous  rocks  in  fissures;  not 
to  be  confounded  with  veins  which  are  precipitated  from  solution. 

Diluvium,  a  name  formerly  applied  to  the  unsorted  and  sorted  de- 
posits of  the  Glacial  period,  as  contrasted  with  the  later  water-sorted 
alluvium.  Compare  Alluvium. 

Diorite,  a  granitoid  rock  consisting  essentially  of  plagioclase  and 
hornblende.  More  or  less  biotite  is  usually  present,  which  may  even 
replace  the  hornblende,  yielding  mica-diorite ;  augite  also  often  appears. 
Acidic  varieties  with  quartz  are  called  quartz-diorites.  See  pp.  59,  64. 
Diorite  is  often  used  as  a  prefix  for  porphyritic  or  other  rocks  related 
to  diorite.  The  name  is  from  the  Greek,  to  distinguish,  in  reference 
to  the  contrasts  of  the  hornblende  and  feldspar,  and  was  given  by  Hauy 
in  1822.  Traite  de  Mineralogie,  IV.,  451.  Diorite  has  been  used  by 
J.  E.  Spurr  as  a  group  name  for  those  igneous  rocks  whose  chief  feldspar 
is  a  plagioclase  belonging  to  the  oligoclase-andesine  series.  2Oth  Ann. 
Rep.  U.  S.  Geol.  Survey,  Part  7,  pp.  204,  209.  There  is  also  a  well- 
marked  disposition  among  petrographers  in  general,  to  imply  a  rock 
with  acidic  plagioclase  by  diorite,  and  one  with  more  basic  varieties  by 
gabbro.  Cf.  H.  W.  Turner,  i;th  Ann.  Rep.  U.  S.  Geol.  Survey,  Part 
I.  730. 

Diorite-porphyrite,  a  porphyrite  whose  groundmass  is  a  finely  crys- 
talline diorite,  and  whose  phenocrysts  are  prevailingly  plagioclase.  It 
is  contrasted  with  hornblende-porphyrite,  whose  phenocrysts  are  pre- 
vailingly hornblende. 

Dipyr,  a  variety  of  scapolite,  often  used  as  a  prefix  to  the  names  of 
rocks  that  contain  the  mineral. 

Disthene,  synonym  of  cyanite,  sometimes  used  as  a  prefix  in  rock 
names. 

Ditroite,  a  nephelite  syenite  from  Ditro  in  Hungary,  especially  rich 
in  blue  sodalite.  See  p.  52. 

Do,  the  first  syllable  of  dominant;  used  as  a  prefix  in  the  nomenclature 
of  the  Quantitative  System  of  Classification.  Thus  dofemic  means 
that  the  femic,  or  ferrogmagnesian  minerals  predominate;  dosalic  that 
the  silica-alumina  minerals,  quartz  and  feldspars  do  the  same.  Various 
other  terms  are  also  employed  which  will  be  found  in  the  treatise  of 
Cross,  Iddings,  Pirsson  and  Washington. 

Dolerite,  coarsely  crystalline  basalts.  The  word  has  had  a  somewhat 
variable  meaning  during  its  history  and  among  different  peoples.  The 


GLOSSARY.  203 

English  use  it  interchangeably  with  diabase ;  indeed  the  definitions  given 
here  justify  this  usage,  except  that  the  characteristic  texture  of  diabase 
is  not  essential  to  this  definition  of  dolerite.  But  the  diabasic  texture 
is  more  of  a  microscopic  feature  than  a  megascopic.  Dolerite  is  from 
the  Greek  for  deceptive,  and  was  given  by  Hauy  in  allusion  to  its  appli- 
cation to  later  rocks,  that  could  not  be  distinguished  from  older  green- 
stones. The  name  is  a  long  standing  indictment  of  the  time  element  in 
the  classification  of  igneous  rocks. 

Dolomite,  is  applied  to  those  rocks  that  approximate  the  mineral  do- 
lomite in  composition.  Named  by  Saussure,  after  Dolomieu,  an  early 
French  geologist.  See  p.  108. 

Dolomitization  or  Dolomization,  the  process  whereby  limestone  be- 
comes dolomite  by  the  substitution  of  magnesian  carbonate  for  a  portion 
of  the  original  calcium  carbonate.  If  the  MgCO3  approximates  the 
45.65  per  cent,  of  the  mineral  dolomite,  there  is  great  shrinkage  in  bulk, 
leading  to  the  development  of  porosity  and  cavities  up  to  n  per  cent,  of 
the  original  rock. 

Domite,  a  more  or  less  decomposed  trachyte  from  the  Puy  de  Dome 
in  the  French  volcanic  district  of  the  Auvergne.  The  typical  domite 
contains  oligoclase  and  is  impregnated  with  hematite. 

Drift,  a  general  name  for  the  unsorted  deposits  of  the  glacial  period. 
Where  subsequently  worked  over  by  water  they  are  called  modified 
drift. 

Dunite,  a  member  of  the  peridotite  group  that  consists  essentially  of 
olivine  and  chromite.  It  was  named  from  the  Dun  Mountains  in  New 
Zealand,  the  original  locality,  but  it  also  occurs  in  North  Carolina. 
The  name  was  given  by  v.  Hochstetter  in  1859.  Geol.  v.  Neu  Seeland, 
218,  1864. 

Durbachite,  a  name  given  to  a  basic  development  at  the  outer  border 
of  a  granite  intrusion  in  Baden.  It  has  the  general  composition  of  mica 
syenite.  The  name  was  given  by  Sauer,  Mitth.  d.  grossh.  bad.  Geol. 
Landesanstalt,  II.,  233. 

Dykes,  see  dikes. 

Dynamometamorphism,  a  general  term  for  those  metamorphic 
changes  in  rocks  that  are  produced  by  mechanical  as  distinguished  from 
chemica  process,  but  the  former  are  seldom  unattended  by  the  latter. 
See  p.  131. 

Dysyntribite,  a  name  given  by  C.  U.  Shepard,  Amer.  Assoc.  Adv. 
Sci.,  1851,  311,  to  a  mineral  or  rock  in  St.  Lawrence  Co.,  N.  Y.,  which 
is  a  hydrated  silicate  of  aluminium  and  potassium,  and  is  related  to 
pinite;  the  same  means  hard  to  crush.  Compare  parophite.  See  also, 


204  A  HAND  BOOK  OF  ROCKS. 

Smith  and  Brush,  Amer.  Jour.  Sci.,  ii.,  XVI.,  50,  and  C.  H.  Smyth 
Jour,  of  Geol.,  II.,  678,  1894. 


Eclogite,  a  more  or  less  schistose  metamorphic  rock,  consisting  of  a 
light-green  pyroxene  (omphacite),  actinolite  (var.  smaragdite)  and 
garnet.  Rarely  found  in  America.  See  p.  143  and  anal.  6,  p.  142 
The  name  is  from  the  Greek  to  select,  in  reference  to  its  attractive  ap- 
pearance. 

Effusive,  a  name  applied  to  those  rocks  that  have  poured  out  in  a 
molten  state  on  the  surface  and  have  there  crystallized,  i.  e.,  volcanic 
rocks.  See  p.  16. 

Ehrwaldite,  a  basic  dike-rock  from  Ehrwald,  consisting  of  pheno- 
crysts  of  altered  olivine,  biotite,  barkevicite  and  titanaugite,  in  a 
groundmassof  augite-microlitesand  altered  glass.  (Rosenbusch,  Mikr. 
Phys.,  II.,  701.  The  name  was  given  by  Pichler.) 

Ekerite,  a  name  given  by  W.  C.  Brogger  to  a  granite,  rich  in  alkalies 
and  having  arfvedsonite  as  its  prevailing  dark  silicate.  (  Nyt.  Mag.  f. 
Naturvid.,  XLIV.,  114,  1906.)  Additional  details  are  given  by  H. 
Rosenbusch.  (Mikros.  Phys.,  4th  ed.,  II.,  525.) 

Elaeolite  or  Eleolite,  a  name  formerly  current  for  the  nephelite  of 
pretertiary  rocks.  It  is  best  known  in  the  rock-name  eleolite-syenite, 
a  synonym  of  nephelite-syenite,  but  the  latter  is  preferable.  See 
nephelite-syenite. 

Elvan,  Cornish  name  for  dikes  of  quartz-porphyry  or  of  granite- 
porphyry. 

Endomorphic,  used  as  a  descriptive  adjective  for  those  phases  of 
contact-metamorphism  that  are  developed  in  the  intrusive  itself.  It  is 
synonymous  with  internal  as  used  on  p.  125. 

Enstatite,  the  variety  of  orthorhombic  pyroxene  with  less  than  5  per 
cent.  FeO.  It  is  largely  used  as  a  prefix  to  the  names  of  rocks  that  con- 
tain the  mineral. 

Eocrystal,  see  under  Brotocrystal. 

Eolian,  fragmental  rocks,  accumulated  through  the  agency  of  the 
wind.  See  p.  99. 

Eorhyolite,  eobasalt,  etc.,  a  series  of  names  proposed  by  O.  Norden- 
skjoeld  for  the  older  equivalents  of  the  rhyolites,  basalts,  etc.  The 
terms  are  practically  equivalent  to  aporhyolite,  apobasalt,  etc.,  but  the 
latter  have  priority.  Bull.  Geol.  Inst.  Univ.  of  Upsala,  I.,  292,  1893. 

Epi,  the  Greek  proposition  for  upon  and  used  as  a  prefix  in  this  sense. 

Epidiabase,  a  name  proposed  by  Issel  as  a  substitute  for  epidiorite 


GLOSSARY.  205 

because  believed  to  be  more  appropriate.  Liguria  geologica,  I.,  324, 
1892.  Cf.  epidiorite. 

Epidiorite,  a  name  applied  to  dikes  of  diabase,  whose  augite  is  in  part 
altered  to  green  hornblende.  The  name  was  coined  before  it  was 
understood  that  the  hornblende  was  secondary  in  this  way.  It  was  first 
applied  by  Giimbel  in  1879  to  a  series  of  narrow  dikes  that  cut  Cambrian 
and  Ordovician  strata  in  the  Fichtelgebirge.  The  name  emphasizes 
their  later  age  than  the  typical  pre-Cambrian  diorites,  but  its  sig- 
nificance has  been  expanded  in  later  years. 

Epidosite,  rocks  largely  formed  of  epidote.  The  epidote  seems 
generally  to  be  produced  by  the  reactions  of  feldspars  and  bisilicates 
upon  each  other  during  alteration. 

Epidote,  the  name  of  the  mineral  is  often  prefixed  to  the  names  of 
rocks  containing  it.  As  a  rule,  the  presence  of  epidote  indicates  the 
advance  of  alteration. 

Epigenetic,  i.  e.,  developed  upon;  specifically  used  for  those  ore- 
deposits  which  are  precipitated  upon  preexisting  rocks,  as  in  veins. 
The  antithesis  is  syngenetic  or  the  ore-bodies  which  are  coincident  in 
origin  with  the  containing  rocks,  such  as  the  basic  segregations  of 
titaniferous  magnetite  in  members  of  the  gabbro  family. 

Equant,  those  textures  of  igneous  rocks  which  consist  of  equidi- 
mensional  crystals.  (Jour.  Geol.,  14:  698,  1906.)  Its  varieties  are 
cuboidal,  polyhedral,  spheroidal,  equant  anhedral,  equant  subhedral. 

Equigranular,  igneous  rocks,  composed  chiefly  of  crystals  of  like 
orders  of  magnitude.  (Jour.  Geol.,  14:  698,  1906.)  The  opposite 
term  is  inequigranular. 

Erlan  or  Erlanfels,  a  name  proposed  by  Breithaupt  for  metamor- 
phic  rocks,  which  consist  essentially  of  augite,  i.  e.,  augite  schists. 
The  name  is  derived  from  the  iron-furnace  at  Erla,  near  Crandorf, 
Saxony. 

Erosion,  geological  term  for  the  process  of  the  removal  of  loose  mate- 
rials in  suspension  in  moving  water  or  in  wind. 

Eruptive,  the  name  ought  properly  to  be  only  applied  to  effusive  or 
volcanic  rocks,  but  it  is  often  used  as  a  synonym  of  igneous. 

Essexite,  a  name  derived  from  Essex  County,  Mass.,  and  applied  by 
J.  H.  Sears  to  a  peculiar  rock,  occurring  with  the  nephelite-syenite  of 
Salem  Neck.  It  is  an  intermediate  rock  among  the  nephelite-syenites, 
the  diorites,  and  the  gabbros,  and  contains  labradorite,  orthoclase,  and 
more  or  less  nephelite  or  sodalite,  together  with  augite,  biotite,  bar- 
kevicite,  olivine,  and  apatite.  Bulletin  Essex  Institute,  XXIII.,  146, 
1891,  H.  S.  Washington,  Journal  of  Geology,  VII.,  53,  1899. 


206  A  HAND  BOOK  OF  ROCKS. 

Esterellite,  a  name  given  by  A.  Michel-Levy  to  a  variety  of  diorite- 
porphyry  from  Esterel,  France.  The  rock  shows  some  peculiarities  of 
chemical  composition  which  have  given  it  special  interest  in  discussions 
relating  to  differentiation.  Bull.  Service  Carte  geol.  de  la  France, 
LVII.,  21,  1897,  Bull.  Soc.  Geol.  de  la  France  (3),  XXIV.,  123. 

Eucrite,  a  name  given  by  G.  Rose  to  rocks  and  meteorites  that  consist 
essentially  of  anorthite  and  augite.  The  term  is  practically  obsolete. 
Pogg.  Annalen,  1835,  I.,  I. 

Eudialyte,  the  name  of  the  mineral  is  sometimes  prefixed  to  the  rare 
nephelite-syenites  that  contain  it. 

Euhedral,  with  well-developed  faces;  applied  to  phenocrysts  or 
smaller  components  of  an  igneous  rock  which  are  completely  bounded 
by  crystal  faces.  (Jour.  Geol.,  14:  698,  1906.)  Much  the  same  as 
automorphic  and  idiomorphic.  The  opposite  term  is  anhedral ;  an  in- 
termediate one  subhedral. 

Euktolite,  a  name  derived  from  the  Greek  words  for  "desired  rock" 
and  given  by  H.  Rosenbusch  to  one  which  filled  a  gap  in  his  classification 
of  rocks.  Sitzungsber.  der  k.  p.  Akad.  Wissensch.,  Berlin,  VII.,  no, 
1899.  The  same  rock  had  been  previously  named  venanzite  (which 
see).  Cf.  Amer.  Jour.  Sci.,  May,  1899,  399. 

Eulysite,  a  name  given  by  Erdmann  to  rocks  interlaminated  with  the 
gneisses  of  Sweden,  and  consisting  of  olivine,  green  pyroxene  and  garnet. 
Neues  Jahrb.,  1849,  837. 

Euphotide,  the  name  chiefly  used  among  the  French  for  gabbro.  It 
was  given  by  Hauy,  and  is  derived  from  the  Greek  words  for  well  and 
light,  in  allusion  to  its  pleasing  combination  of  white  and  green. 

Eurite,  used  among  the  French  as  a  synonym  of  felsite,  but  also  ap- 
plied to  compact  rocks,  chiefly  feldspar  and  quartz,  such  as  some  granu- 
lites.  The  name  was  first  given  by  Daubisson  to  the  groundmass  of 
porphyries,  because  of  their  easy  fusibility  compared  with  hornstone 
or  flint. 

Eutaxitic,  a  general  name  for  banded  volcanic  rocks.  The  banding 
is  due  to  the  parallel  arrangement  of  portions  of  the  rock  that  are  con- 
trasted either  in  mineralogy  or  texture. 

Eutectic,  a  term  now  used  by  both  petrologists  and  metallographers, 
for  those  molten  or  solidified  mixtures  of  two  or  more  compounds  or 
metals,  which  when  considered  as  solutions  one  of  the  other,  mutually 
saturate  each  other  or  one  another.  In  alloys,  when  two  metals  are 
melted  together,  the  fusing  point  of  the  combination  is  lower  than  the 
fusing  point  of  either.  By  suitable  proportions  a  mixture  is  finally 
reached  which  has  the  lowest  possible  fusing  point.  The  same  is  true 


GLOSSARY.  207 

of  minerals.  If  now  we  imagine  such  a  mixture  in  a  molten  condition 
and  cooling,  it  will  remain  molten  down  to  the  minimum  temperature. 
On  passing  this  mark  both  must  crystallize  simultaneously  and  yield 
interpenetrating  or  at  least  intimately  mixed  crystals.  These  are 
called  eutectics.  Graphic  granite  is  the  most  familiar  illustration  in 
rocks.  It  contains  approximately  25  per  cent,  quartz  and  75  per  cent. 
orthoclase. 

Evergreenite,  a  name  derived  from  the  Evergreen  mine,  at  Apex, 
Gil  pin  Co.,  Colo.,  and  given  by  E.  A.  Ritter  to  a  dike  of  general  granitoid 
texture,  but  with  micropegmatitic  and  porphyritic  phases.  The 
principal  minerals  are  quartz,  orthoclase,  albite  (both  at  times  forming 
microperthite),  aegirite,  enstatite  and  diallage.  The  dike  also  contains 
chal  copy  rite  and  bornite  as  original,  rock-making  minerals  and  coor- 
dinate in  amount  with  the  normal  components.  The  copper  minerals 
give  the  dike  economic  value.  The  dike  is  thus  practically  a  nord- 
markite  with  added  metallic  sulphides.  Bull.  Amer.  Inst.  Min.  Eng., 
No.  19:  33,  1908. 

Exomorphic,  a  descriptive  term  for  those  changes  which  are  produced 
by  contact-metamorphism  in  the  wall  rock  of  the  intrusion;  the  antithesis 
of  endomorphic.  It  is  synonymous  with  external  as  used  in  p.  125. 

Extrusive,  synonym  of  effusive,  much  used  in  America. 


Fabric,  the  shape  and  arrangement  of  the  crystalline  and  non- 
crystalline  parts  of  an  igneous  rock.  (Jour.  Geol.,  14:  693,  1906.) 
Together  with  crystallinity  and  granularity  (size  of  components) 
fabric  completes  the  three  features  of  texture. 

Fahlband,  a  name,  originally  applied  in  Norway,  to  bands  of  meta- 
morphic  rocks,  which  are  impregnated  with  metallic  sulphides,  such 
that  the  out-crop  is  rusty,  the  name  meaning  foul  or  rotten  layer.  The 
Fahlbands  are  of  especial  importance  in  the  Kongsberg  region,  where 
they  exercise  an  enriching  influence  upon  the  veins  which  cross  them. 

Farrisite,  a  name  derived  from  Lake  Farris  in  Norway,  and  applied 
by  Brogger  to  a  very  peculiar  rock,  which  is  as  yet  known  only  in  one 
small  dike.  The  rock  is  finely  granular  in  texture  and  consists  of  some 
soda-bearing,  but  not  sharply  identified,  tetragonal  mineral  related  to 
melilite,  together  with  barkevicite,  colorless  pyroxene,  biotite,  serpen- 
tinous  pseudomorphs  after  olivine,  magnetite  and  apatite.  Das  Gang- 
gefolge  des  Laurdalits,  66,  1898. 

Feldspar,  the  name  of  the  mineral  is  often  prefixed  to  the  names 
of  those  rocks  that  contain  it,  such  as  feldspar-porphyry,  feldspar- 
basalt,  etc. 


208  A  HAND  BOOK  OF  ROCKS. 

Feldspathoids,  silicates  of  alumina  and  an  alkali  or  alkaline  earth, 
that  are  practically  equivalent  to  feldspars  in  their  relations  in  rocks. 
The  principal  ones  are  nephelite,  leucite  and  melilite,  but  sodalite, 
nosean,  hauyne,  analcite  and  even  muscovite  could  perhaps  be  also 
considered  such,  although  their  composition  may,  in  instances,  vary 
from  the  above. 

Felsite,  the  word  was  first  applied  in  1814  by  Gerhard,  an  early  geol- 
ogist, to  the  fine  groundmasses  of  porphyries.  These  were  recognized  to 
be  fusible  as  distinguished  from  hornstone,  which  they  resembled  (com- 
pare eurite).  Felsite  is  now  especially  used  for  those  finely  crystalline 
varieties  of  quartz-porphyries,  porphyries  or  porphyrites  that  have  few 
or  no  phenocrysts,  and  that,  therefore,  give  but  slight  indications  to 
the  unaided  eye  of  their  actual  mineralogical  composition.  The  mi- 
croscope has  shown  them  to  be  made  up  of  microscopic  feldspars, 
quartzes  and  glass.  Petrosilex  has  been  used  as  a  synonym.  See  p.  17. 

Felsitic,  has  been  employed  as  a  megascopic  term  in  the  preceding 
pages  to  describe  those  textures  which  are  characteristic  of  felsites,  i.  e., 
micro-crystalline,  but  without  phenocrysts.  See  p.  17.  It  is  often 
used  also  to  describe  the  groundmasses  of  truly  porphyritic  rocks,  that 
are  micro-crystalline,  but  clearly  not  glassy.  In  this  sense  we  have 
felsite-porphyry,  felso-liparite,  felso-dacite,  etc. 

Felsophyre,  a  contraction  for  felsite-porphyry. 

Felspar,  the  current  spelling  of  feldspar  among  the  English.  It  is 
based  on  an  old  typographical  error  in  Kirwan's  Mineralogy,  I.,  317, 
1794,  now,  however,  firmly  established  in  general  usage. 

Femic,  a  very  useful  term  coined  by  the  authors  of  the  Quantitative 
System,  from  the  first  letters  of  ferrum  amd  magnesium,  as  used  in 
ferromagnesian.  Femic  means  therefore  rich  in  ferromagnesian 
minerals,  as  is  the  case  with  the  basic  igneous  rocks.  Its  antithesis 
is  salic. 

Fergusite,  a  name  derived  from  Fergus  Co.,  Montana,  and  coined  by 
L.  V.  Pirsson  to  describe  a  granitoid  intrusive  rock  consisting  of  dom- 
inant leucite,  now  represented  by  pseudoleucite  and  subordinate  augite. 
The  accessories  are  apatite,  iron  ores,  biotite,  and  sporadic  olivine. 
Fergusite  is  the  deep-seated  representative  of  the  leucitites.  The 
typical  locality  is  the  Arnoux  stock  of  the  Highwood  mountains. 
(Bulletin  237,  U.  S.  Geol.  Survey,  89,  1905.) 

Ferrite,  microscopic  crystals  of  iron  oxide. 

Ferrolite,  Wadsworth's  name  for  rocks  composed  of  iron  ores.  Rept. 
State  Geol.  Mich.,  1891-92,  p.  92. 

Fibrolite,  synonym  of  sillimanite  and  sometimes  used  as  a  prefix  to 
rock  names. 


GLOSSARY.  209 

Fiorite,  siliceous  sinter,  named  from  Mt.  Santa  Fiora,  in  Tuscany. 

Firn,  Swiss  name  for  the  granular,  loose  or  consolidated  snow  of  the 
high  altitudes  before  it  forms  glacial  ice  below. 

Flaser-structure,  a  structure  developed  in  granitoid  rocks  and  espe- 
cially in  gabbros  by  dynamic  metamorphism.  Small  lenses  of  granular 
texture  are  set  in  a  scaly  aggregate  that  fills  the  interstices  between 
them.  It  appears  to  have  been  caused  by  shearing  that  has  crushed 
some  portions  more  than  others,  and  that  has  developed  a  kind  of  rude 
flow-structure. 

Flint,  a  compact  and  crypto-crystalline  aggregate  of  chalcedonic  and 
opaline  silica.  Chert  and  hornstone  are  synonyms.  See  pp.  116,  117. 

Float,  a  term  much  used  among  Western  miners  for  loose,  surface 
deposits,  which  are  usually  somewhere  near  their  parent  ledges. 

Flow-structure,  a  structure  due  to  the  alignment  of  the  minerals  or 
inclusions  of  an  igneous  rock  so  as  to  suggest  the  swirling  curves,  eddies 
and  wavy  motions  of  a  flowing  stream.  It  is  caused  by  the  chilling  of 
a  flowing,  lava  current.  Fluxion-structure  is  synonymous. 

Foliation,  the  banding  or  lamination  of  metamorphic  rocks  as  dis- 
tinguished from  the  stratification  of  sediments. 

Forellenstein,  a  variety  of  olivine-gabbro,  consisting  of  plagioclase, 
olivine  and  more  or  less  pyroxene.  The  dark  silicates  are  so  arranged 
in  the  lighter  feldspar  as  to  suggest  the  markings  of  a  trout.  (German, 
Forelle.) 

Formation,  as  defined  and  used  by  the  U.  S.  Geological  Survey,  is  a 
large  and  persistent  stratum  of  some  one  kind  of  rock.  It  is  also  loosely 
employed  for  any  local  and  more  or  less  related  group  of  rocks.  In 
Dana's  Geology  it  is  applied  to  the  groups  of  related  strata  that  were 
formed  in  a  geological  period. 

Fortunite,  a  dike  rock  from  Fortuna  in  the  Spanish  province  of  Murcia, 
originally  named  by  Ram6n  Adan  de  Yarza.  (Bol.  Com.  del  mapa 
geol.  de  Espana,  20,  1893.)  In  a  glassy  groundmass  are  found  pheno- 
crysts  of  olivine,  phlogopite  and  apatite,  together  with  so-called  "  be- 
lonites."  The  supposed  absence  of  pyroxene  led  the  author  to  separate 
it  from  Verite.  A.  Osann  later  identified  the  belonites  as  diopside,  and 
demonstrated  by  chemical  analysis  that  verite  and  fortunite  are  the 
same  species.  Osann's  analyses  also  prove  the  phlogopite  to  be  prac- 
tically of  the  same  composition  as  that  from  the  Leucite  Hills,  Wyo. 
(Festschrift  zum  siebzigsten  Geburtstage  von  Harry  Rosenbusch, 
1906,  263.) 

Fourchite,  a  name  proposed  by  J.  Francis  Williams  for  those  basic 
dike  rocks  that  consist  essentially  of  augite  in  a  glassy  groundmass,  i.  e., 
14 


210  A  HAND  BOOK  OF  ROCKS. 

dike-augitites.  The  name  was  suggested  by  Fourche  Mountain,  Ark., 
where  they  are  abundant.  Ann.  Rep.  Geol.  Sur.  Ark.,  1890,  II.,  107. 

Foyaite,  a  name  originally  applied  to  the  nephelite  syenite,  with 
supposed  hornblende,  of  Mt.  Foya  in  the  Monchique  range  of  Portugal. 
Although  the  hornblende  has  since  proved  to  be  augite  and  segirite,  the 
name  foyaite  is  still  employed  for  nephelite  syenite  with  hornblende. 
See  p.  52. 

Fragmental,  a  descriptive  term  for  the  rocks,  such  as  sandstones 
and  breccias,  which  are  formed  from  fragments  of  preexisting  rocks. 
Clastic  is  synonymous. 

Fraidronite,  a  name  used  by  early  French  geologists  for  a  variety  of 
minette. 

Freestone,  a  quarryman's  name  for  those  sandstones  that  submit 
readily  to  tool  treatment. 

Fruchtschiefer,  German  name  for  a  variety  of  spotted,  contact  schists 
in  the  outer  zone  of  the  aureole.  See  p.  127. 

Fuller's  earth,  a  fine  earth,  resembling  clay,  but  lacking  plasticity. 
It  is  much  the  same  chemically  as  clay,  but  has  a  decidedly  higher  per- 
centage of  water. 

Fulgurite,  little  tubes  of  glassy  rocks  that  have  been  fused  from  all 
sorts  of  other  rocks  by  lightning  strokes.  They  are  especially  frequent 
on  exposed  crags  or  mountain  tops.  The  name  is  derived  from  the 
Latin  for  thunderbolt. 

Q 

Gabbro,  an  Italian  word  formerly  used  for  a  rock  composed  of  serpen- 
tine and  diallage.  It  was  later  applied  to  igneous  rocks,  of  granitoid 
texture,  consisting  of  plagioclase  and  diallage,  but  as  now  employed, 
any  monoclinic  pyroxene  may  be  present,  with  or  without  diallage.  As 
the  name  of  a  group,  it  includes  in  addition  to  the  rocks  with  mono- 
clinic  pyroxene,  those  with  plagioclase  and  orthorhombic  pyroxene  as 
well ;  and  even  the  peridotites  and  pyroxenites,  from  their  close  geo- 
logical connection  with  the  gabbros  may  conveniently  be  embraced. 
Although  of  the  same  mineral  composition  with  gabbro,  yet  the  peculiar 
and  contrasted  texture  of  diabase  may  be  remarked.  Intermediate 
types  have  even  been  called  gabbro-diabase.  See  p.  78.  A  full  review 
of  the  meaning  and  history  of  gabbro,  by  W.  S.  Bayley,  will  be  found 
in  Jour,  of  Geology,  I.,  435,  Aug.,  1893. 

Gabbro-diorite,  gabbro  with  hornblende  which  may,  in  fact,  be 
secondary  after  augite.  Intermediate  rocks  between  true  gabbros  and 
diorites. 

Ganggesteine,  German  for  dike  rocks. 


GLOSSARY.  211 

Garewaite,  a  porphyritic  and  very  basic  dike-rock,  from  the  northern 
Urals.  Phenocrysts  of  diopside  are  found  in  a  ground  mass,  chiefly 
olivine,  magnetite  and  chromite.  Minor  components  are  pyroxene  and 
labradorite.  (L.  Duparc  and  F.  Pearce,  Comptes  Rendus,  CXXXIX., 
154,  1894.) 

Garganite,  a  name  suggested  by  Viola  and  de  Stefani  for  a  dike  rock 
in  the  Italian  province  of  Foggia,  which  in  the  middle,  with  prevailing 
alkali-feldspar  contains  both  augite  and  amphibole,  i.  e,,  is  a  vogesite; 
on  the  edges  it  contains  biotite,  hornblende  and  olivine,  and  resembles 
kersantite.  Boli.  Roy.  Com.  Geol.  d' Italia,  1893,  129. 

Garnet-rock,  a  rock  composed  essentially  of  garnets. 

Gauteite,  a  name  derived  from  the  Gaute  valley,  central  Bohemia, 
and  given  by  J.  E.  Hibsch  to  a  leucocratic  dike  rock  of  porphyritic 
texture  and  trachytic  habit.  The  phenocrysts  are  hornblende,  augite, 
and  abundant  lime-soda  feldspar.  The  groundmass  is  about  80  per 
cent,  feldspar  rods,  with  the  remainder  magnetite  grains,  small  horn- 
blendes, augites,  biotites  and  a  little  colorless  glass.  The  gauteite  is 
regarded  as  a  complementary  dike-rock  to  neighboring  camptonites  and 
is  believed  to  correspond  to  the  deep-seated  monzonites.  Tsch.  Mitth., 
XVII.,  87,  1897. 

Generations  of  minerals  in  an  igneous  rock  refer  to  the  groups  of 
individuals  that  crystallize  out  at  a  definite  period  and  in  a  more  or  less 
definite  succession  during  cooling.  The  same  species  may  have  one, 
two,  or  very  rarely  three  generations.  See  p.  21. 

Geodes,  hollow,  rounded  boulders  lined  with  crystals  proiecting  in- 
ward from  the  walls. 

Geest,  a  name  proposed  by  J.  A.  DeLuc  in  1816  for  "the  immediate 
products  of  rock  decay  in  situ."  It  is  a  provincial  word  for  earth  in 
Holland  and  northern  Germany.  Abreg6  Geologique,  Paris,  1816, 
112,  as  quoted  by  W  J  McGee,  nth  Ann.  Rep.  U.  S.  Geol.  Survey, 
Part  I.,  279.  Compare  Laterite,  Saprolite. 

Geyserite,  siliceous  deposits  from  a  geyser.     See  p.  116. 

Gieseckite-porphyry,  a  nephelite  porphyry  from  Greenland,  whose 
nephelite  phenocrysts  are  altered  to  the  aggregate  of  muscovite  scales, 
which  was  called  gieseckite  under  the  impression  that  it  was  a  new 
mineral.  Liebenerite  porphyry  is  the  same  thing  from  Predazzo,  in  the 
Tyrol. 

Giumarrite,  a  dike-rock,  described  as  a  hornblende-bearing  augitite, 
and  occurring  near  Giumarra,  Sicily.  (C.  Viola,  Boll.  Roy.  Com.  geol. 
d'ltalia,  1901.) 

Gladkaite,   a  dike-rock  in  the  dunite  of  the  Gladkaia  Sopka,  northern 


212  A  HAND  BOOK  OF  ROCKS. 

Urals.  It  has  a  finely  crystalline  texture  and  consists  of  acidic  plagio- 
clase,  quartz,  hornblende,  biotite,  apatite,  magnetite  and  secondary 
epidote  and  muscovite.  (L.  Duparc  and  F.  Pearce  Comptes  Rendus, 
CXL.,  1614,  1905.  See  also  Nature,  June  22,  1905,  p.  192.) 

Glass,  the  amorphous  result  of  the  quick  chill  of  a  fused  lava.  See 
pp.  25,  26,  27,  and  for  glassy  texture,  p.  16. 

Glauconite,  the  green  silicate  of  iron  and  potassium  that  is  important 
in  many  green  sands.  See  p.  103. 

Glaucophane,  a  blue  soda-amphibole  found  especially  in  certain,  rare 
schists.  See  pp.  142,  143. 

Globulite,  spheroidal  beginnings  of  crystals;  believed  by  Vogelsang, 
to  be  the  first  recognizable  elements  of  crystals,  which  could  be  observed 
with  high  powers  especially  when  the  crystallizing  process  was  retarded 
by  some  thick  medium  such  as  glycerine. 

Glomeroporphyritic,  a  textural  term  proposed  by  Tate  for  those  por- 
phyritic  rocks  whose  feldspar  phenocrysts  are  made  up  of  an  aggregate 
of  individuals  instead  of  one,  large  crystal.  British  Assoc.  Adv.  Sci., 
1890,  814.  Compare  Ocellar. 

Gneiss,  a  laminated  or  foliated  granitoid  rock  that  corresponds  in 
mineralogical  composition  to  some  one  of  the  plutonic  rocks.  The  name 
originated  among  the  Saxon  miners.  See  p.  133. 

Granite,  in  restricted  signification  is  a  granitoid,  igneous  rock  con- 
sisting of  quartz,  orthoclase,  more  or  less  oligoclase,  biotite  and  musco- 
vite, but  is  widely  used  in  a  more  general  sense.  The  first  three  may 
also  be  combined  with  either  of  the  micas  alone,  with  hornblende  or 
with  augite.  In  its  technical  applications  as  a  name  of  a  building  stone 
it  is  used  for  almost  any  crystalline  rock  composed  of  silicates,  as  con- 
trasted with  sandstones,  slates,  limestones  and  marbles.  It  is  a  very  old 
term.  See  pp.  33-38. 

Granitelle,  a  granite  with  comparatively  little  mica,  so  that  it  con- 
sists almost  entirely  of  quartz  and  feldspar;  binary  granite.  It  has  been 
also  used  by  R.  D.  Irving  for  augite-granite.  U.  S.  Geol.  Surv.,  Mono- 
graph V.,  p.  115. 

Granite-Porphyry,  practically  a  quartz-porphyry  with  a  coarsely 
crystalline  groundmassand  preponderating  phenocrysts;  an  intermediate 
rock  between  granites  and  typical  quartz-porphyries,  having  the  same 
minerals  as  the  former,  but  being  porphyritic  like  the  latter.  The  chief 
phenocrysts  are,  however,  feldspars.  See  p.  31. 

Granitite,  a  special  name  for  biotite-granite.  It  is  much  the  com- 
monest of  the  granites. 

Granitoid,  used  in  preceding  pages  as  a  textural  term  to  describe 


GLOSSARY.  213 

those  igneous  rocks  which  are  entirely  composed  of  recognizable  min- 
erals of  approximately  the  same  size.  It  was  suggested  by  granite,  the 
most  familiar  of  the  rocks  which  show  this  characteristic.  See  p.  17. 
In  the  granitoid  texture  each  kind  of  mineral  appears  in  but  one  gener- 
ation, and  the  individuals  seldom  have  crystal  boundaries. 

Granodiorite,  a  term  which  has  been  given  special  currency  by  the 
usage  of  the  U.  S.  Geological  Survey,  and  which  is  employed  for  the 
intermediate  rocks  between  granites  and  quartz-diorites.  It  is  a  con- 
traction for  granite-diorite  and  is  a  very  useful  rock  name.  Compare 
Adamellite. 

Granolite,  a  name  suggested  by  L.  V.  Pirsson  for  the  igneous  rocks- 
described  as  granitoid  on  preceding  pages.  It  is  a  shortening  of  gran- 
ular rock  and  embraces  the  plutonic  series  from  granite  through  peri- 
dotite.  (See  Jour.  Geol.,  7:  141,  1899.) 

Granophyre,  a  descriptive  term  used  in  connection  with  microscopic 
study,  to  describe  those  groundmasses  in  quartz-porphyries  and  micro- 
granites  in  which  the  quartz  and  feldspar  crystals  have  simultaneously 
crystallized,  so  as  to  mutually  penetrate  each  other.  The  several  parts- 
of  one  individual,  though  separated  from  one  another,  extinguish  to- 
gether between  crossed  nicols.  Micropegmatitic  is  synonymous.  See 
also  micro-perthitic,  micro-poicilitic,  and  micro-granitic. 

Granulite,  properly  speaking  a  finely  crystalline,  laminated,  meta- 
morphic  rock  consisting  essentially  of  orthoclase,  quartz  and  garnet, 
but  having  also  at  times  cyanite,  hornblende,  biotite  or  augite.  The 
name  means  garnet  rock  (i.  e.,  German  for  garnet,  Granat).  The 
granulites  are  best  developed  in  the  mountains  of  Saxony.  Sometimes 
the  name  is  less  correctly  used  for  muscovite  granite,  or  for  granites 
containing  little  else  than  quartz  and  feldspar.  See  p.  136. 

Graphic  Granite,  an  old  name  for  that  variety  of  pegmatite  consisting 
of  bladed  crystals  of  quartz  contained  in  orthoclase.  When  cracked 
across  the  long  axes  of  the  quartzes,  the  V-shaped  and  spindle-shaped 
cross-sections,  suggest  the  cuneiform  inscriptions  of  the  ancient  alpha- 
bets, hence  the  name.  (See  p.  35.)  The  structure  is  due  to  the  simul- 
taneous crystallization  of  the  two  minerals.  The  quartzes  neighboring 
to  one  another  are  parts  of  a  single  crystal.  The  containing  orthoclase 
is  a  single  crystal.  The  aggregate  is  one  of  the  best  available  instances 
of  an  eutectic.  (See  Eutectic.)  The  same  structure  is  developed 
microscopically  and  is  called  micro-pegmatitic,  or  granophyric,  or  simply 
micrographic. 

Graphite,  the  name  of  the  mineral  is  often  prefixed  to  the  names  of 
rocks  containing  it,  as  graphite-gneiss,  graphite-schist,  etc. 


214  A  HAND  BOOK  OF  ROCKS. 

Graywacke,  an  old  name  of  loose  signification,  but  chiefly  applied  to 
metamorphosed,  shaly  sandstones  that  yield  a  tough,  irregularly  break- 
ing rock,  different  from  slate  on  the  one  hand  and  from  quartzite  on  the 
other.  The  components  of  graywacke  may  be  largely  bits  of  rocks, 
rather  than  fragments  of  minerals. 

Greenschists,  chlorite  schists,  which  may,  however,  be  of  quite 
diverse  origin.  See  p.  142. 

Greenstone,  an  old  field  name  for  those  compact,  igneous  rocks  that 
have  developed  enough  chlorite  in  alteration  to  give  them  a  green  cast. 
They  are  mostly  diabases  and  diorites.  Greenstone  is  partially  synony- 
mous with  trap.  It  is  often  used  as  a  prefix  to  other  rock  names. 

Greisen,  a  granitoid  but  often  somewhat  cellular  rock,  composed  of 
quartz  and  muscovite  or  some  related  mica,  rich  in  fluorine.  It  is  the 
characteristic  mother  rock  of  the  ore  of  tin,  cassiterite,  and  is  in  most 
cases  a  result  of  the  contact  action  of  granite  and  its  evolved  mineral- 
izers.  See  p.  130. 

Grit,  coarse  sandstone. 

Grorudite,  Brogger's  name  for  a  porphyritic,  dike  rock  from  Grorud, 
near  Christiania,  Norway.  The  phenocrysts  are  microcline  and  aegirite ; 
the  groundmass  consists  of  rectangular  orthoclase,  quartz  and  aegirite. 
It  is  a  variety  of  granite  porphyry.  Zeitsch.  f.  Krys.,  XVI.,  65. 

Groundmass,  the  relatively  finely  crystalline  or  glassy  portion  of  a 
porphyritic  rock  as  contrasted  with  its  phenocrysts.  Not  to  be  con- 
founded with  basis,  as  will  be  seen  by  referring  to  the  latter.  On 
groundmass,  see  p.  17. 

Grass,  the  fragmental  products  of  the  weathering  of  granite  in  its 
passage  to  soil. 

Guano,  phosphatic  accumulations,  useful  as  fertilizers  and  chiefly 
derived  from  the  excrement  of  sea-fowl,  deposited  on  marine  islands. 

Gumbo,  a  name  current  in  Western  and  Southern  States  for  those 
soils  that  yield  a  sticky  mud  when  wet. 

Gypsum,  the  name  of  the  mineral  is  also  applied  to  its  bedded  deposits, 
which  constitute  sedimentary  rocks  in  series,  usually  associated  with 
rock-salt. 

H 

Halleflinta,  a  Swedish  name  for  dense,  compact  metamorphic  rocks, 
consisting  of  microscopic  quartz,  and  feldspar  crystals,  with  occasional 
phenocrysts  and  sometimes  hornblende,  chlorite,  magnetite  and  hema- 
tite. They  are  associated  with  gneisses,  but  are  of  obscure  origin. 

Haloidite,  Wadsworth's  name  for  rock-salt.  Kept.  State  Geol.  Mich., 
1891-92,  p.  92. 


GLOSSARY.  21 5 

Haplite,  a  name  proposed  by  L.  Fletcher  for  that  variety  of  granite 
which  consists  of  quartz  and  potash  feldspar.  The  name  is  derived  from 
the  Greek  for  simple.  An  Introduction  to  the  Study  of  Rocks.  British 
Museum  Guide-books,  1895,  58,  63,  102.  Compare  Binary  granite. 

Hard-pan,  a  name  specially  developed  in  the  digging  of  auriferous 
placers,  and  applied  to  the  layers  of  gravel  which  are  usually  present  a 
few  feet  below  the  surface  and  which  are  cemented  by  limonite  or  some 
similar  bond.  They  are  therefore  resistant.  It  is  also  used  to  describe 
boulder-clay,  which  is  likewise  difficult  to  excavate. 

Harrisite,  Marker's  name  for  a  granitoid  igneous  rock  with  olivine 
predominant  over  anorthite.  Compare  allivalite.  The  two  are  inter- 
mediate between  peridotites  and  anorthosites.  The  name  is  derived 
from  a  locality  in  the  Hebrides. 

Harzburgite,  a  name  proposed  by  Rosenbusch  for  those  peridotites 
that  consist  essentially  of  olivine  and  enstatite  or  bronzite.  Mass. 
Gest.,  1887,  269.  Saxonite  was  earlier  proposed  by  VVadsworth  (1884) 
for  the  same  rock,  and  has  priority. 

Hatherlite,  a  name  proposed  by  A.  Henderson  for  a  syenite  from 
South  Africa  which  has  for  its  feldspar  anorthoclase  instead  of  ortho- 
clase.  Petrographical  and  Geological  Investigations  of  certain  Transvaal 
Norites,  Gabbros,  Pyroxenites,  etc.,  1898.  Pilandite  is  a  porphyritic 
phase  of  the  same. 

Hauyne,  the  name  of  the  mineral  is  often  prefixed  to  the  names  of 
those  rocks  that  contain  it,  as  hauyne-basalt,  hauyne-trachyte,  etc. 

Hedrumite,  a  name  proposed  by  Brogger  for  certain,  syenitic  rocks 
that  are  poor  or  lacking  in  nephelite,  but  that  have  a  trachytic  texture. 
Zeitschr.  Krys.,  XVI.,  40.  Das  Ganggefolge  des  Laurdalits,  183. 

Hemithrene,  Brogniart's  name,  current  among  the  French,  for  cer- 
tain dioritic  rocks  that  have  a  large  amount  of  calcite,  presumably  an 
alteration  product. 

Heptorite,  a  variety  of  monchiquite,  containing  in  a  colorless,  iso- 
tropic  groundmass,  assumed  to  be  a  glass,  basaltic  augite,  acicular  horn- 
blende and  hauynite.  It  occurs  as  a  finely  crystalline,  narrow  dike  on 
the  contact  of  trachyte  and  graywackes  in  the  Siebengebirge  or  Seven 
Mountains  of  the  Rhine  valley.  The  name  is  given  by  K.  Busz  and  is 
based  on  the  Greek  equivalent  of  Siebengebirge.  (Neues  Jahrbuch, 
1904,  II.,  86.) 

Heronite,  a  name  proposed  by  A.  P.  Coleman,  for  a  dike  rock,  con- 
sisting essentially  of  analcite,  orthoclase,  plagioclase  and  aegirite,  the 
analcite  having  the  character  of  a  base,  in  which  the  other  minerals 
form  radiating  groups  of  crystals.  The  name  is  derived  from  the  lo- 


216  A  HAND  BOOK  OF  ROCKS. 

cality,  Heron  Bay,  on  the  north  shore  of  Lake  Superior.  Journal  of 
Geology,  VII.,  1899,  431. 

Heumite,  a  name  proposed  by  W.  C.  Brogger  for  a  dike  rock,  com- 
posed of  minerals,  too  small  to  be  recognized  with  the  eye  alone,  but 
which  under  the  microscope  prove  to  be  natronorthoclase,  natronmicro- 
cline,  barkevicite,  biotite,  and  in  small  amount,  nephelite,  sodalite  and 
diopside.  The  accessories  are  apatite,  magnetite,  pyrite  and  titanite. 
The  silica  in  two  dikes  was  found  to  be  respectively  47.10  and  48.46. 
The  name  was  derived  from  Heum,  a  small  town  on  Lake  Farris.  Das 
Ganggefolge  des  Laurdalits,  90,  1898. 

Hiatal,  see  seriate. 

Holocrystalline,  a  textural  term  applied  to  those  rocks  that  consist 
entirely  of  crystlalized  minerals  as  distinguished  from  those  with  more 
or  less  glass. 

Holyokeite,  a  name  derived  from  Mt.  Hoi  yoke,  Mass.,  and  suggested 
by  B.  K.  Emerson  for  an  almost  purely  feldspathic  phase  of  the  Triassic 
diabase,  found  in  fragments  in  a  volcanic  breccia.  The  rock  is  70  per 
cent,  albite,  with  calcite  (16.42),  orthoclase  (9.41),  and  ilmenite  (1.63), 
as  the  principal  remaining  constituents.  (Journal  of  Geology,  X.,  508, 
1902.) 

Hornblende,  the  name  of  the  mineral  is  prefixed  to  many  rock  names. 

Hornblendite,  a  granitoid,  igeneous  rock  consisting  essentially  of 
hornblende  and  analogous  to  pyroxenite.  See  p.  83. 

Hornfels,  a  dense,  compact  rock  produced  from  slates  by  the  contact 
action  of  some  igneous  intrusion,  especially  granite.  Various  micro- 
scopic minerals  are  developed  in  it.  See  p.  126. 

Hornstone,  synonym  of  flint  and  chert. 

Horses,  a  miner's  term  for  fragments  of  wall  rock  included  in  a  vein. 

Hudsonite,     See  cortlandtite. 

Hyaline,  a  synonym  of  glassy,  which  is  often  prefixed  to  the  name  of 
volcanic  rocks  to  signify  a  glassy  development,  as  hyalo-rhyolites. 

Hyalomelane,  Hausmann's  name  for  basaltic  glass.  The  word  is 
derived  from  the  Greek  for  black  glass. 

Hybrid,  Harker's  name  for  igneous  rocks,  which  result  from  the 
commingling  of  two  distinct  magmas  or  from  the  absorption  of  solid 
rocks  by  molten  masses.  They  are  contrasted  with  the  products  of 
differentiation.  (Natural  History  of  the  Igenous  Rocks,  333,  1909.) 

Hydato,  a  syllable  prefixed  to  lithological  terms  to  indicate  an  origin 
through  aqueous  processes. 

Hydatopneumatolithic,  a  term  used  in  the  discussion  of  certain  ore 
deposits  to  describe  their  origin  through  the  agency  of  water  and  vapors. 


GLOSSARY.  217 

Hydrogenic,  a  general  name  proposed  by  A.  W.  Grabau  for  the  rocks 
which  are  due  to  the  agency  of  water.  It  is  somewhat  more  literally 
comprehensive  of  the  precipitates  from  solution,  such  as  rock  salts, 
stalagmitic  marbles,  etc.,  than  is  sedimentary.  It  forms  a  logical  member 
of  the  sequence,  pyrogenic;  hydrogenic;  atmogenic. 

Hypabyssai,  Brogger's  term  for  those  igneous  rocks  whose  textures 
are  transitional  between  the  granitoid  varieties  of  the  deep-seated  and 
the  typical  porphyritic  or  glassy  varieties  of  the  effusive.  Dikes  are 
the  commonest  illustrations  but  border  facies  of  large  masses  are  also 
included.  The  components  tend  to  be  automorphic  and  if  porphyritic 
the  rock  is  fully  crystallized.  (Eruptivgest.der  Krist.  Geb.,  1 : 123,  1894.) 

Hypautomorphic,  see  under  idiomorphic. 

Hyperite,  used  in  Sweden  loosely  for  the  rocks  of  the  gabbro  family, 
and  in  a  restricted  sense  for  olivine-norite. 

Hypersthenite,  a  somewhat  obsolete  name  for  norite. 

Hypidiomorphic,  see  under  idiomorphic. 

Hypohyaline,  a  partly  glassy  texture  in  an  igneous  rock.  (Jour. 
Geol.,  14:  693,  1906.) 

Hysterobase,  a  name  given  by  K.  A.  Lassen  to  the  rock  of  a  series 
of  dikes,  related  to  the  diabases,  but  differing  from  them,  in  often  having 
quartz,  brown  biotite,  and  brown  hornblende,  the  last  sometimes 
replacing  the  augite.  There  may  be  also  some  glass  basis.  Zeitschr. 
d.  d.  g.  Gesellsch.,  XL.,  204,  1888. 

Hysterogenite,  Posepny's  term  for  mineral  deposits  derived  from  the 
debris  of  other  rocks.  The  word  means  of  subsequent  or  later  deposition 
than  in  their  original  homes.  Trans.  Amer.  Inst.  Min.  Eng.,  XXIII., 
211.  Compare  idiogenite,  xenogenite. 

Hysteromorphous,  a  term  suggested  by  Posepny  for  those  ore  de- 
posits that  have  been  formed  from  some  other  original  deposits  by  the 
chemical  and  mechanical  influences  of  the  surface  region.  Trans.  Amer. 
Inst.  Min.  Eng.,  XXIII,  331,  1893. 


Idiogenites,  a  term  suggested  by  Posepny  to  describe  those  ore  de- 
posits which  are  contemporaneous  in  origin  with  the  wall  rock.  The 
word  means  of  the  same  origin.  Trans.  Amer.  Inst.  Min.  Eng.,  XXIII., 
211,  1893.  Compare  xenogenite,  hysterogenite. 

Idiomorphic,  a  descriptive  term  for  those  component  minerals  of  a 
rock  that  have  their  own  crystal  faces.  Rarely  all  are  of  this  character, 
and  then  the  rock  is  called  panidiomorphic.  Again,  some  are,  and 
others  are  not,  giving  the  hypidiomorphic  texture.  The  phenocrysts  of 


218  A  HAND  BOOK  OF  ROCKS. 

porphyritic  rocks  are  most  prone  to  be  idiomorphic.  When  no  minerals 
have  their  own  crystal  faces,  as  in  most  granites,  the  rock  is  allotrio- 
morphic,  as  earlier  explained.  All  these  terms  were  suggested  by 
Rosenbusch,  Mass.  Gest.,  1887,  but  Rohrbach's  automorphic  and 
xenomorphic,  as  is  stated  under  the  former,  have  a  year's  priority  and 
mean  the  same  thing.  The  words  are  of  chief  importance  in  micro- 
scopic work. 

Ijolite,  a  granitoid,  nephelite  rock,  occurring  in  Finland  and  corre- 
sponding in  mineralogy  to  the  nephelinites.  It  contains  chiefly  nephe- 
lite and  pyroxene.  The  name  is  derived  from  the  lijoki  River,  Fin- 
land, and  was  given  by  Ramsay  and  Berghell.  Stockholm  geol.  foren. 
forh.,  1891,  300. 

Ilmenite  is  sometimes  prefixed  to  those  rocks  which  contain  enough  of 
the  mineral  to  receive  attention  as  ores:  thus  ilmenite-gabbro,  limenite- 
norite,  etc.  See  J.  H.  L.  Vogt,  Zeitschrift  prakt.  Geologic,  I.,  n, 
1893. 

Imandrite,  a  name  suggested  by  W.  Ramsay  for  a  contact  rock,  be- 
lieved to  have  been  produced  from  graywacke  by  the  neighboring 
nephelite-syenite  of  Umptek.  The  original  feldspars  are  now  largely 
silicified.  (Cited  by  Rosenbusch,  Mikr.  Phys.,  4th  ed.f  II.,  251.) 

Inclusions,  the  term  is  applied  to  crystals  and  anhedra  of  one  min- 
eral involved  in  another;  and  to  fragments  of  one  rock  inclosed  in 
another,  as  when  a  volcanic  flow  picks  up  portions  of  its  conduit. 

Infusorial  earth.     See  diatomaceous  earth. 

Intersertal,  applied  to  those  microscopic  textures  of  igneous  rocks 
which  have  in  the  interstices  of  well-developed  minerals,  residual 
masses  of  glass,  i.  e.,  undifferentiated  magma. 

Intratelluric,  a  term  applied  to  those  processes  that  take  place  deep 
within  the  earth.  For  example,  the  large  phenocrysts  of  a  porphyry  are 
usually  of  intratelluric  cystallization.  See  p.  21. 

Intrusive,  the  contrasted  term  with  effusive,  and  applied  to  those 
rocks  that  have  crystallized  without  reaching  the  surface.  They  there- 
fore form  dikes,  laccoliths  and  batholiths.  Plutonic  is,  to  a  certain 
extent,  synonvmous.  See  p.  16. 

Itabirite,  a  metamorphic  rock,  first  described  from  Brazil,  of  schis- 
tose structure  and  composed  essentially  of  quartz  grains  and  scales  of 
specular  hematite.  Some  muscovite  is  also  present.  It  is  a  close  rela- 
tive of  itacolumite.  It  was  named  from  Itabira,  a  place  in  Brazil. 
When  it  crumbles  to  powder  it  is  called  jacotinga.  Heusser  and  Claraz, 
Zeit.  d.  d.  geol.  Gesellsch.,  XI.,  448,  1859. 

Itacolumite,  or  flexible  sandstone,  is  a  peculiar  quartz  schist  first  de- 


GLOSSARY.  219 

scribed  from  Brazil,  but  since  found  in  North  Carolina  and  elsewhere. 
It  is  composed  of  quartz  grains,  to  whose  interlocking  it  is  supposed  to 
owe  its  flexibility ;  of  muscovite,  talc  and  a  few  other  minerals,  and  has 
been  regarded  as  the  mother  rock  of  the  Brazilian  diamonds.  See  p.  144. 

J 

Jacupirangite,  a  name  derived  from  Jacupiranga,  Prov.  Sao  Paulo, 
Brazil,  and  applied  by  O.  A.  Derby  to  a  group  of  igneous  rocks,  con- 
sisting sometimes  of  pure  magnetite;  again  of  magnetite  with  accessory 
pyroxene;  or  of  pyroxene  with  accessory  magnetite;  or  of  pyroxene 
and  nephelite  and  biotite  and  olivine  in  greater  or  less  quantity.  Amer. 
Jour.  Sci.,  April,  1891,  314. 

Jasper,  red  chalcedony,  abundant  enough  on  Lake  Superior  and 
elsewhere  to  be  a  rock. 

Jasperoid,  resembling  jasper. 

Jaspilite,  a  name  originally  proposed  by  Wadsworth  for  all  the  acid 
eruptive  rocks,  whose  chemical  and  physical  condition  carries  them 
above  the  rhyolites,  but  now  used  more  or  less  loosely  around  Lake 
Superior  for  the  jasper  associated  with  the  local  iron  ores. 

Josefite,  a  name  given  by  Szadeczky,  to  a  peridotite  occurring  in 
dikes  at  Assuan,  Egypt.  Fold.  Kozl.,  XXIX.,  210, 1899.  There  seems 
to  be  no  good  reason  for  the  new  name.  Cf.  Tsch.  Mitth.,  XIX.,  169. 

K 

Kakirite,  a  name  derived  from  Lake  Kakir  in  Swedish  Lapland  and 
given  by  F.  Svenonius,  to  extremely  sheared  and  crushed  rocks  which 
are  found  in  Lapland  along  thrust  faults.  The  rocks  are  so  badly 
crushed  and  altered  that  their  original  character  cannot  be  deciphered. 
(See  Eleventh  Internat.  Geol.  Congress  Guide-book  I.,  No.  6,  p.  26, 
1910.) 

Kaolinite,  the  hydrated  silicate  of  alumina,  AUOs,  2SiC>2,  2H2O,  that  is 
the  base  of  clays  and  that  gives  them  plasticity.  When  kaolinite  is 
mingled  with  varying  amounts  of  comminuted  quartz,  and  yields  a 
pure  white  clay,  the  mixture  is  kaolin.  See  p.  101. 

Katamorphic.     See  Anamorphic. 

Katzenbuckelite,  a  name  suggested  by  A.  Osann  for  the  famous 
nephelite-porphyry  or  biotite-tinguaite  porphyry  of  Katzenbuckel, 
Baden.  Phenocrysts  of  nephelite,  biotite,  olivine,  noselite  and  mag- 
netite are  set  in  a  coarser  or  finer  groundmass  of  nephelite,  biotite  and 
sometimes  segirite  and  amphibole.  (Rosenbusch,  Mikros.  Phys.,  4th 
ed.,  II.,  632.)  The  rock  is  almost  the  same  thing  as  the  nephelite- 
porphyry  or  sussexite  (which  see)  of  Beemerville,  N.  J. 


220  A  HAND  BOOK  OF  ROCKS. 

Kedabekite,  a  name  given  by  E.  von  Federow  to  a  dike  rock  from 
the  Kedabek  mines,  government  of  Elizabethpol,  Transcaucasia.  The 
rock  is  finely  granular,  dark  gray  in  color  and  consists  of  basic,  plagio- 
clase,  lime-iron,  garnet  and  a  pleochroic  pyroxene  called  violaite.  Re- 
view in  Amer.  Jour.  Sci.,  Sept.,  1901,  247. 

Kelyphite-rim,  a  name  applied  by  Schrauf  to  rims  of  pyroxene,  horn- 
blende and  spinel  that  sometimes  surround  the  garnets  of  peridotites. 
It  is  used  in  the  microscopic  description  of  rocks. 

Kentallenite,  a  name  based  on  the  Kentallen  quarry  in  Argyllshire, 
Scotland,  and  given  by  Hill  and  Kynaston,  to  a  basic  member  of  the 
monzonite  family,  which  contains  chiefly,  augite  and  olivine,  and  less 
abundantly,  biotite,  orthoclase,  and  plagioclase,  the  last  two  in  some- 
what variable  relations.  The  rocks  are  very  high  in  magnesia.  Quar. 
Jour.  Geol.  Soc.,  LVL,  537,  1900.  Cf.  shonkinite. 

Kenyte,  a  name  given  by  J.  W.  Gregory  to  "  liparitic  representatives 
of  an  olivine-bearing  nepheline  syenite,  consisting  of  anorthoclase 
phenocrysts  and  a  glassy  or  hyalopilitic  groundmass,  which  varies 
in  color  from  grayish  green  to  a  deep  sepia  brown.  ^Egyrine,  if  present, 
occurs  in  small  granules;  aenigmatite  and  quartz  are  absent."  The 
rocks  occur  as  surface  flows  on  Mt.  Kenya  in  East  Africa  and  are 
practically  phonolites  with  a  glassy  groundmass.  Quar.  Jour.  Geol. 
Soc.,  LVL,  211,  1900.  Amer.  Jour.  Sci.,  Sept.,  1901,  247. 

Keratophyre,  a  rock  intermediate  between  porphyries  and  porphy- 
rites,  and  differing  from  either  in  having  as  the  principal  feldspar, 
anorthoclase  instead  of  either  orthoclase  or  the  soda-lime  feldspars. 
Keratophyre  applies  to  pretertiary  rocks,  whereas  pantellerite  is  used 
for  the  same  aggregate  of  more  recent  geological  date.  The  name 
was  given  in  1874  by  Giimbel  to  certain  Bavarian  felsitic  and  porphy- 
ritic  rocks,  that  resembled  hornfels,  hence  the  name  from  the  Greek  for 
horn.  Its  significance  has  since  been  restricted. 

Kern,  the  German  word  for  core,  introduced  in  a  special  sense  into 
petrology  by  H.  Rosenbusch,  in  the  development  of  the  Kern-theorie. 
(Tsch.  Mitth.,  ii :  144,  1889.)  The  author  tabulates  a  series  of  rock 
analyses,  representing  the  various  types  of  igneous  rocks.  These  are 
recast  to  molecular  proportions,  and  then  to  atomic  proportions,  alike 
of  silicon  and  the  metallic  elements.  Combinations  of  these  are  next 
developed  which  are  called  Kerns,  and  which  are  believed  to  be  char- 
acteristic of  the  different  igneous  types,  such  as  granites,  syenites, 
nephelite  syenites,  etc. 

Kersantite,  a  very  old  name  of  somewhat  varying  application,  but 
formerly  used  for  rocks  that  are  intermediate  between  diorites  or  their 


GLOSSARY.  221 

corresponding  porphyrites  and  gabbros  or  diabases.  Mica-diabase 
was  used  as  a  synonym.  Rosenbusch,  in  carrying  out  the  separation 
of  the  dike  rocks  from  the  effusive  and  intrusive  grand  divisions,  has 
sought  to  restrict  the  name  to  those  dike  rocks  with  plagioclase  that  have 
prevailing  dark  silicates  of  which  the  chief  is  biotite.  Kersanton  is 
practically  a  synonym.  Both  names  are  derived  from  a  town  in 
Brittany. 

Kies,  a  general  term  for  the  sulphide  ores,  now  adopted  into  English 
from  the  original  German. 

Kieselguhr,  German  name  for  diatomaceous  earth,  and  more  or  less 
current  in  English. 

Killas,  Cornish  miner's  term  for  the  slates  or  schists  that  form  the 
country  rock  of  the  Cornish  tin  veins. 

Kimberlite,  a  name  given  by  H.  Carville  Lewis  to  the  peridotite,  that 
in  large  part  forms  the  diamantiferous  neck  at  the  Kimberly  mines,  of 
South  Africa.  (Geol.  Magazine,  1887,  22.)  The  rock  is  more  porphy- 
ritic  than  typical  peridotite. 

Kinzigite,  a  metamorphic  rock  consisting  of  biotite,  garnet  and 
oligoclase.  It  was  named  in  1860  by  Fischer,  from  the  Kinzig  Valley, 
in  the  Black  Forest.  Neues  Jahrb.,  1860,  796. 

Knotty,  a  descriptive  term  for  those  slates  or  schists,  which  are  so 
altered  by  contact  metamorphism  as  to  have  new  minerals  developed 
in  them,  giving  them  a  spotted  or  knotty  appearance.  See  p.  127. 

Kodurite,  a  name  derived  from  the  Kodur  manganese  mine  near 
Vizagapatam,  in  the  northeastern  portion  of  the  Madras  Presidency, 
India.  It  was  given  by  L.  L.  Fermor  to  a  rock  consisting  of  potash 
feldspar,  manganese  garnet,  and  apatite.  It  is  usually  of  granitoid 
texture,  with  medium  coarseness  of  grain,  but  it  may  be  pegmatitic. 
(Geol.  Survey  of  India,  XXXV.) 

Koellite,  a  name  suggested  by  W.  C.  Brogger  for  a  basic  dike-rock, 
consisting  of  olivine,  lepidomelane,  barkevicite,  apatite,  magnetite, 
anorthoclase  and  nephelite.  (H.  Rosenbusch,  Mikros.  Phys.,  4th  ed., 

IL,  70S-) 

Koswite,  a  name  derived  from  Mt.  Koswimsky  in  the  Urals  and 
given  by  Duparc  and  Pearce,  to  a  melanocratic,  granular  rock  composed 
of  varieties  of  pyroxene,  olivine,  hornblende,  chromiferous  spinels  and 
magnetite;  the  last-named  constituting  a  matrix  or  cement  for  the 
others.  Mem.  Soc.  Phys.  et  d'Hist.  nat.  de  Geneve,  XXXIV.,  218. 
Amer.  Jour.  Sci.,  Jan.,  1903,  84. 

Krablite,  ejected  blocks  from  the  volcano  of  Krafla,  in  Iceland,  which 
were  regarded  many  years  ago  by  Forchhammer,  under  the  name 


222  A  HAND  BOOK  OF  ROCKS. 

baulite,  as  a  feldspar,  of  percentage  in  silica  far  beyond  that  of  albite- 
(Jour.  f.  prakt.  Chemie,  1843,  390;  Jahresber.  iiber  die  Fortschrit. 
Chemie  u.  Mineralogie,  1844,  262.)  It  was  soon  shown  by  the  micro. 
scope  to  be  an  aggregate. 

Krageroite,  a  gabbroic  rock,  consisting  of  plagioclase  and  rutile  and 
occurring  at  Kragero,  Norway.  (H.  Rosenbusch,  Mikr.  Phys.,  4th 
ed.,  II.,  354.)  Compare  nelsonite. 

Krassyk,  a  local  name  for  a  decomposed  ferruginous  schist  —  in  the 
Beresov  gold-mining  district  of  the  Urals.  Archiv  fur  practische 
Geologic,  II.,  537. 

Kugel,  the  German  word  for  ball  or  sphere  and  often  prefixed  to  those 
igneous  rocks  that  show  a  spheroidal  development,  such  as  corsite, 
orbicular  granite,  etc. 

Kulaite,  a  name  derived  from  the  Kula  basin  in  Lydia,  Asia  Minor, 
proposed  by  H.  S.  Washington,  for  those  rare  basalts  (there  abundant) 
in  which  hornblende  surpasses  augite  in  amount.  "  The  Volcanoes  of 
the  Kula  Basin."  Privately  printed.  New  York,  1894,  Amer.  Jour. 
Sci.,  Feb.,  1894,  p.  115. 

Kullaite,  a  name  derived  from  the  Swedish  locality  Kullen,  and 
applied  by  A.  Hennig  to  a  dike-rock  which  is  regarded  as  an  intermediate 
type  between  the  diabases  and  the  granites.  In  a  feldspathic  ground- 
mass  of  ophitic  (diabasic?)  texture,  are  red  phenocrysts  of  plagioclase 
and  microcline.  The  groundmass  has  rods  of  oligoclase-andesine  with 
augite,  orthoclase  and  titaniferous  magnetite.  See  Review  in  Neues 
Jahrbuch,  1901,  II.,  59. 

Kuskite,  a  name  derived  from  the  Kuskokwim  river,  Alaska,  and 
applied  by  J.  E.  Spurr  to  certain  porphyritic  dikes,  which  cut  Cretaceous 
shales,  and  which  have  phenocrysts  of  quartz,  scapolite,  and  probably 
basic  plagioclase  (the  last  now  represented  by  alteration  products),  in  a 
groundmass  of  quartz,  orthoclase  and  muscovite.  Compare  yentnite, 
Amer.  Jour.  Sci.,  Oct.,  1900,  311  and  315. 

Kyschtymite,  a  name  derived  from  the  Kyschtym  mining  district  of 
the  Urafs,  and  given  by  J.  Morozewicz  to  a  rock  consisting  chiefly  of 
anorthite  and  corundum,  with  which  are  associated  biotite,  spinel, 
zircon,  apatite  and,  as  secondary  minerals,  muscovite,  chlorite,  kaolinite 
and  chromite,  Tsch.  Mitth.,  XVIII.,  212,  1898. 


Labile,  the  condition  which  is  reached  by  a  cooling  solution  when 
crystallization  spontaneously  takes  place.  It  is  in  a  measure  con- 
trasted with  metastable.  (See  Iddings,  Igneous  Rocks,  i  :  160,  1910.) 


GLOSSARY.  223 

Labradorite,  the  name  of  the  feldspar  is  prefixed  to  many  rock  names. 
Labradorite  rock  was  formerly  much  used  for  anorthosite,  which  see. 

Laccolite,  a  name  based  on  the  Greek  word  for  cistern  and  suggested 
by  G.  K.  Gilbert  for  those  intrusions  of  igneous  rock  that  spread  out 
laterally  between  sedimentary  beds  like  a  huge  lens,  and  that  never 
reach  the  surface  unless  exposed  by  erosion.  See  p.  15;  also  Geology 
of  the  Henry  Mountains,  Utah,  p.  19. 

Lamprophyre,  a  general  term,  now  used  in  a  somewhat  wider  sense 
than  as  originally  proposed  by  Gurnbel,  who  suggested  it.  Rosenbusch, 
in  the  Massigen  Gesteine,  gave  it  its  present  significance.  Lam- 
prophyres  are  dike  rocks  of  porphyritic  texture,  whose  predominant 
phenocrysts  are  the  dark  silicates,  augite,  hornblende  or  biotite.  They 
are  practically  basic  dikes.  The  word  means  a  shining  rock,  and  was 
first  applied  in  1874  to  small  dikes  in  the  Fichtelgebirge  that  were  rich 
in  biotite.  In  a  somewhat  modified  sense  it  has  recently  been  employed 
by  L.  V.  Pirsson,  as  single  term  for  the  basic  "  complementary  rocks  " 
(see  Complementary  Rocks),  and  as  the  antithseis  of  oxyphyre,  which 
applies  to  the  acidic  complementary  rocks  of  an  eruptive  area. 

Lapilli,  volcanic  dust  and  small  ejectments,  the  results  of  explosive 
eruptions. 

Lassenite,  Wadsworth's  name  for  unaltered,  glassy  trachytes. 
Rept.  State  Geol.  Mich.,  1891-92,  p.  97.  The  name  is  derived  from 
Lassen's  Peak,  Cal. 

Laterite,  a  name  derived  from  the  Latin  word  for  brick  earth,  and 
applied  many  years  ago  to  the  red,  residual  soils  or  surface  products, 
that  have  originated  in  situ  from  the  atmospheric  weathering  of  rocks. 
They  are  especially  characteristic  of  the  tropics.  Though  first  applied 
to  altered,  basaltic  rocks  in  India,  laterite  has  had  in  later  years  a 
general  application  without  regard  to  the  character  of  the  original  rock. 
Compare  saprolite.  See  pp.  154,  155. 

Latite,  a  name  suggested  by  F.  L.  Ransome,  for  the  rocks  that  are 
intermediate  among  the  trachytes,  andesites  and  basalts.  Latite  is 
meant  to  be  a  broad  family  name  and  to  include  the  effusive  repre- 
sentatives of  the  plutonic  monzonites.  Plagioclase  and  orthoclase  are 
both  present;  augite,  hornblende,  biotite  and  olivine  vary  in  relative 
amounts.  The  textures  may  be  glassy,  felsitic  or  porphyritic.  The 
name  is  derived  from  the  Italian  province  of  Latium  but  was  sug- 
gested by  studies  on  Table  Mtn.,  Tuolumne  Co.,  Calif.,  Bull.  89,  U.  S. 
Geol.  Survey.  Compare  trachydolerite,  ciminite,  vulsinite,  monzonite. 

Laurdalite,  a  name  given  by  Brogger  to  a  coarsely  crystalline  variety 
of  nephelite-syenite,  that  is  abnormal  in  having  for  its  feldspar  natron- 


224  A  HAND  BOOK  OF  ROCKS. 

orthoclase,  rarely  natron-microcline,  instead  of  the  normal  potash  ortho- 
clase.  The  dark  silicates  are  biotite,  diallage  and  olivine.  Zeitsch.  f. 
Kryst.,  XVI.,  28,  1890. 

Laurvikite,  a  name  applied  by  Brogger  to  a  Norwegian  variety  of 
augite-syenite  that  contains  natron-orthoclase  as  its  chief  feldspar  and 
most  abundant  mineral.  The  other  components  are  rare  plagioclase, 
pyroxene,  biotite,  barkevicite  or  arfvedsonite,  olivine  and  magnetite: 
Besides  microscopic  accessories,  nephelite  is  occasionally  met.  Zeitsch. 
f.  Kryst.,  XVI.,  29,  1890.  Compare  pulaskite. 

Lava,  a  general  name  for  the  molten  outpourings  of  volcanoes. 

Laxite,  Wadsworth's  name  for  the  fragmental  or  mechanical  rocks, 
especially  when  unconsolidated.  Kept,  of  State  Geol.  of  Mich.,  1891- 
92,  p.  98. 

Lenneporphyry,  consolidated  tuffs  in  the  Devonian  of  Westphalia 
and  having  the  composition  of  quartz-keratophyres.  The  name  is 
derived  from  the  Lenne,  a  stream  along  which  they  occur. 

Lentils,  a  short  name  for  lenticular  beds  in  a  stratified  series. 

Leopardite,  a  siliceous  rock  from  North  Carolina,  spotted  with  stains 
of  manganese  oxide.  It  is  usually  considered  to  be  a  quartz-porphyry. 

Leopard  rock,  a  local  name  in  Canada,  applied  to  pegmatitic  rocks 
which  are  associated  with  the  apatite  veins  of  Ontario  and  Quebec. 
See  C.  H.  Gordon,  Bulletin  Geolog.  Society  of  America,  VII.,  122. 

Leptinite  or  Leptynite,  the  French  synonym  of  granulite  as  used 
among  the  Germans.  See  granulites. 

Leptite,  a  term  now  widely  used  by  the  students  of  the  Precambrian 
formations  in  Sweden  for  all  the  very  finely  schistose  and  close-grained 
rocks,  such  as  have  been  previously  called  halleflintas,  eurites,  por- 
phyroids,  fine  phyllites,  etc.  (A.  Hogbom,  Bull.  Geol.  Inst.  Univ.  of 
Upsala,  10:  42,  1910.) 

Leptomorphic,  a  term  suggested  by  Giimbel  for  crystallized  substances 
which  lack  definite  crystalline  borders,  as  the  nephelite  in  many  ground- 
masses.  Fichtelgebirge,  1879,  240. 

Lestiwarite,  a  name  proposed  by  Rosenbusch  for  the  aplitic  dike- 
rocks  that  accompany  nephelite-syenites  in  Norway  and  Finland.  They 
are  chiefly  or  almost  entirely  alkali  feldspar,  with  very  subordinate 
pyroxene  or  amphibole.  They  have  been  previously  called  syenite- 
aplites  by  W.  C.  Brogger.  Lestiwarite  is  derived  from  the  Finnish 
locality  Lestiware.  Massige  Gesteine,  II.,  464.  Das  Ganggefolge  des 
Laurdalits,  207. 

Leucite,  the  name  of  the  mineral  is  prefixed  to  the  names  of  many 
rocks  which  contain  it,  as,  leucite-absarokite,  leucite-syenite,  etc. 


GLOSSARY.  22$ 

Leucite-basalt,  basaltic  rocks  with  olivine,  in  which  leucite  replaces 
plagioclase.  See  p.  72. 

Leucite-basanite,  basaltic  rocks  that  contain  both  leucite  and  pla- 
gioclase. As  contrasted  with  leucite-tephrites,  they  contain  olivine. 
See  p.  72. 

Leucitite,  basaltic  rocks  without  olivine  in  which  leucite  replaces 
plagioclase.  Compare  leucite-basalt. 

Leucitophyre,  a  name  formerly  used  as  a  general  one  for  the  leucite 
rocks,  but  now  by  common  consent  restricted  to  those  phonolites  that 
contain  both  leucite  and  nephelite. 

Leucite-tephrite,  basaltic  rocks  without  olivine,  that  contain  both 
plagioclase  and  leucite.  Compare  leucite-basanite. 

Leucocratic,  a  descriptive  term,  suggested  by  W.  C.  Brogger  for 
those  eruptive  rocks  in  which  the  light-colored  minerals,  i.  e.,  the  feld- 
spars, feldspathoids  and  quartz,  are  in  excess  over  the  dark-colored 
(ferromagnesian)  minerals.  Leucocratic  is  derived  from  two  Greek 
words  meaning  ''white  prevails."  The  antithetical  term  is  melano- 
cratic. 

Leucophyre,  originally  applied  by  Gumbel  in  1874  to  light-colored 
diabases  whose  feldspar  was  altered  to  saussurite  and  whose  augite  had 
largely  changed  to  chlorite.  Rosenbusch  restricts  it  to  diabases  poor  in 
plagioclase.  The  name  means  a  light-colored  or  white  porphyritic 
rock,  and  has  little  claim  to  consideration  either  in  etymology  or  appli- 
cation. 

Lherzolite,  a  variety  of  peridotite,  first  discovered  in  the  Pyrenees, 
and  containing  olivine,  diopside  and  an  orthorhombic  pyroxene.  Much 
picotite  is  also  present.  It  was  named  from  Lake  Lherz,  by  de  la 
Metherie,  Theorie  de  la  Terre,  II.,  281. 

Liebenerite-porphyry,  nephelite-porphyry  whose  nephelite  pheno- 
crysts  are  altered  to  muscovite.  Its  original  locality  is  near  Predazzo, 
in  the  Tyrol.  Compare  gieseckite-porphyry. 

Limburgite,  porphyritic  basaltic  rocks  consisting  of  olivine  and 
augite  in  a  glassy  groundmass.  They  lack  feldspars.  See  p.  73.  The 
name  is  derived  from  Limburg,  a  locality  on  the  Kaiserstuhl,  a  basaltic 
mountain  in  Baden.  It  was  suggested  by  Rosenbusch  in  1872,  and  at 
the  same  time  Boricky  described  similar  rocks  from  Bohemia  as  magma- 
basalt. 

Limestone,  the  general  name  for  rocks  composed  essentially  of  cal- 
cium carbonate.  See  p.  105. 

Limurite,  a  name  for  a  rock  consisting  of  axinite,  pyroxene,  amphi- 
bole,  quartz,  titanite,  calcite,  pyrite  and  pyrrhotite,  which  occurs  on 
15 


226  A  HAND  BOOK  OF  ROCKS. 

the  contact  of  granite  and  limestone,  although  formerly  thought  to  be  a 
member  of  the  crystalline  schists.  A.  Lacroix,  Comptes  rendus,  CXIV., 
955,  1892. 

Lindoite,  Brogger's  name  for  certain  dike  rocks,  in  the  region  of 
Kristiania.  They  have  trachytic  texture;  are  seldom  and  then  but 
slightly  porphyritic;  are  medium  to  coarsely  crystalline  in  the  larger 
dikes;  possess  light  colors  and  often  lack  dark-colored  minerals.  When 
such  are  recognizable  they  are  pyrite  and  chlorite.  Ferriferous  carbo- 
nates are  present.  Traces  of  segirite  and  of  a  dark,  alkaline  hornblende 
may  be  occasionally  detected.  (Die  Eruptivgesteine  des  Kristianiage- 
bietes,  I.,  131,  1894.) 

Linophyric,  having  the  phenocrysts  of  a  porphyritic  rock  in  lines. 
(Jour.  Geol.,  14:  703,  1906.) 

Liparite,  a  synonym  of  rhyolite,  and  largely  used  for  the  latter 
among  Europeans,  though  rhyolite  is  chiefly  current  in  America  and 
England.  The  name  is  derived  from  the  Lipari  Islands,  off  the  coast  of 
Italy,  where  the  rocks  are  abundant.  It  was  proposed  by  Justus  Roth 
in  1861.  Gesteins-analysen,  p.  xxxiv. 

Listvenite,  a  local  name  for  a  rock  in  the  gold-mining  district  of 
Beresov,  in  the  Urals.  It  is  regarded  as  a  contact  zone  produced  from 
dolomite,  and  is  a  coarsely  crystalline  aggregate  of  magnesite,  talc, 
quartz  and  limonite,  pseudomorphic  after  pyrite.  Archiv  fur  prac- 
tische  Geologic,  II.,  437. 

Litchfieldite,  a  name  proposed  by  W.  S.  Bayley  for  the  variety  of 
nephelite-syenite,  occurring  near  Litchfield,  Maine,  in  loose  boulders 
whose  chief  feldspar  is  albite  and  which  differ  therein  from  normal 
nephelite-syenite.  Bull.  Geol.  Soc.  Amer.,  III.,  243. 

Lithical,  a  term  proposed  by  L.  Fletcher  for  the  finer,  textural  char- 
acters of  rocks,  i.  e.,  those  for  which  texture,  as  distinguished  from  struc- 
ture, is  employed  above.  See  p.  16.  Lithical,  from  the  Greek  for  stone, 
is  contrasted  with  petrical,  from  the  Greek  for  rock.  Introduction  to 
the  Study  of  Rocks;  British  Museum  Handbooks,  1895. 

Lithionite-granite,  a  name  proposed  by  Rosenbusch  for  granites  with 
lithia  mica  or  lithionite. 

Lithographic  limestone,  an  exceptionally  homogeneous  and  fine- 
grained limestone,  suitable  for  lithography. 

Lithoidal,  a  descriptive  term  applied  to  those  groundmasses,  espe- 
cially of  rhyolites,  that  are  excessively  finely  crystalline,  like  porcelain, 
as  distinguished  from  glassy  varieties.  The  English  equivalent,  stony, 
is  also  used. 

Lithology,  that  branch  of  geology  which  makes  rocks  a  special  object 


GLOSSARY.  227 

of  study.  Practically  synonymous  with  petrology:  the  former  being 
derived  from  the  Greek  for  stone;  the  latter  from  the  Greek  for  rock. 
Lithology  was  current  30-50  years  ago  and  has  been  to  a  large  degree 
displaced  by  petrology  or  petrography  since  1890. 

Lithophysae,  literally  "stone  bubbles,"  a  name  applied  to  those 
cellular  cavities  in  acidic  lavas,  obsidian,  rhyolite,  etc.,  that  have  con- 
centric walls,  and  that  are  caused  by  a  special  development  of  mineral- 
izers  at  that  particular  point.  They  were  usually  hemispherical  in 
shape  and  on  the  walls  may  have  various  well  crystallized  minerals. 
See  pp.  26,  27. 

Lithosphere,  the  outer  stony  shell  of  the  earth.     See  barysphere. 

Local  metamorphlsm,  *.  e.,  contact  metamorphism.     See  p.  122. 

Loess,  fine  surface  soils  chiefly  formed  of  wind-blown  dust.  See  p. 
99.  The  name  is  a  German  word,  akin  to  loose,  and  appears  to  have 
been  first  applied  geologically  in  the  Rhine  valley. 

Luciite,  Chelius'  name  from  the  Luciberg  in  Hesse,  for  finely  crystal- 
line, diorite  dikes,  whose  minerals  are  xenomorphic.  Notizblatt  Verein. 
f.  Erdkunde,  Darmstadt,  1892,  i. 

Luijaurite,  a  name  proposed  by  Brogger  for  a  nephelite-syenite,  rich 
in  aegirite  and  eudialyte.  Zeitsch.  f.  Kryst.,  XVI.,  204.  The  name 
is  from  a  Lapland  locality,  where  the  rock  was  discovered  by  Ramsay. 

Lustre-mottlings,  a  name  applied  by  Pumpelly  to  certain  augitic 
rocks,  which  have  a  shimmering  lustre  because  the  shining  cleavage 
faces  of  the  augite  crystals  are  mottled  by  small  inclusions.  Proc. 
Amer.  Acad.,  XIII.,  260,  1878.  Compare  Poicilitic  and  Schiller. 

Luxullianite,  a  tourmaline  granite  from  Luxullian,  in  Cornwall,  that 
is  a  product  of  contact  metamorphism.  See  p.  35. 

Lutite,  a  general  name,  suggested  by  A.  W.  Grabau  for  finely  frag- 
mental  sediments,  such  as  shales,  clays,  argillaceous  limestones,  etc. 
Prefixes  define  the  kind  of  sediment,  such  as  silicolutite,  calcilutite  and 
the  like.  (Bull.  Geol.  Soc.  Amer.,  14:  348-352.)  The  adjective  is 
lutaceous. 

Lydite.     See  Basanite. 

M 

Macroscopic,  a  word  formerly  current  as  a  synonym  of  megarcopic, 
i.  e.,  recognizable  by  the  naked  eye.  It  is  etymologically  less  correct 
as  an  antithesis  of  microscopic  than  is  megascopic,  for  "macro"  is 
from  the  Greek  for  broad,  whereas  "mega"  means  large.  Neverthe- 
less, it  preceded  megascopic  in  general  use  and  is  still  current. 

Madupite,  a  name  given  by  Whitman  Cross  to  a  peculiar  group  of 
rocks  which  are  illustrated  by  one  forming  Pilot  Knob,  a  mesa  about  6 


228  A  HAND  BOOK  OF  ROCKS. 

miles  northwest  of  Rock  Springs,  Wyo.  Cross  defines  Madupite  "as 
consisting  essentially  of  diopside  and  a  magnesia-potash  mica  with 
leucite  in  decidedly  subordinate  amount.  Its  magma  is  low  in  silica, 
alumina  and  iron,  rich  in  potash,  and  contains  so  much  lime  and  mag- 
nesia that  silicates  of  these  bases  are  the  principal  constituents,  yet 
controlled  in  their  development  by  the  strong  potash  element."  The 
Pilot  Knob  case  is  a  vitrophyric  representative  of  the  type,  so  defined. 
Amer.  Jour.  Sci.,  Aug.,  1897,  139. 

Masnaite,  a  name  derived  from  Lake  Meena,  near  Gran,  Norway, 
and  given  by  W.  C.  Brogger  to  an  intrusive  trachytic  rock,  regarded  as 
a  differentiation  product  of  a  gabbro-magma.  Maenite  is  a  bostonite 
relatively  rich  in  lime  and  poor  in  potash.  Erupt.  Gest.  Krist.,  III., 
207,  1899. 

Magma  is  now  generally  employed  for  the  molten  masses  of  igneous 
rock  before  they  have  crystallized.  An  original,  parent  magma  may 
break  up  into  several  derived  ones,  a  process  called  magmatic  dif- 
ferentiation. See  p.  20.  Magma  is  also  used  in  the  sense  of  basis  as 
earlier  defined,  but  this  use  is  unfortunate. 

Magma-basalt,  a  synonym  of  limburgite,  which  was  proposed  by 
Boricky,  in  1872,  at  about  the  same  time  that  Rosenbusch  suggested 
limburgite.  Some  authorities  give  the  former  the  preference. 

Magnetite,  the  name  of  the  mineral  is  prefixed  to  the  names  of  many 
rocks  in  which  it  is  prominent.  It  almost  furnishes  a  rock  itself,  at 
times. 

Magnetite-olivinite,  a  name  coined  by  A.  Sjogren  in  1876  for  the 
igneous  iron-ore  at  Taberg,  in  Sweden.  The  rock  is  an  aggregate  of 
magnetite  and  olivine,  with  a  few  shreds  of  biotite.  The  rock  is  prac- 
tically a  peridotite,  greatly  enriched  with  titaniferous  magnetite.  On 
the  borders  of  the  intrusion  it  shades  into  gabbro.  Geol.  Foren.  in 
Stockholm,  Forhandl.,  III.,  42.  Compare  Cumberlandite. 

Magnetite-spinellite,  an  eruptive  iron  ore  occurring  at  Routivara, 
Sweden,  and  consisting  of  magnetite  (in  part  titaniferous),  spinel,  and 
smaller  amounts  of  olivine,  pyroxene,  apatite  and  pyrrhotite.  The  ore 
contains  about  14  per  cent,  titanic  oxide.  W.  Petersson,  Geol.  Foren. 
Forh.,  XV.,  49,  i893- 

Magnophyric,  coarsely  porphyritic  phenocrysts  greater  than  5  mm. 
(Jour.  Geol.,  14:  702, 1906.)  If  spelt  magniphyric,  the  texture  is  coarse 
only  in  a  microscopic  sense;  phenocrysts  .2-.O4  mm. 

Malchite,  a  variety  of  diorite  dikes  which  have,  in  a  groundmass  of 
quartz,  feldspar  and  hornblende,  phenocrysts  of  plagioclase,  hornblende 
and  biotite.  The  name  was  given  by  A.  Osann,  and  is  derived  from 
Malchen,  another  name  for  Mt.  Melibocus,  in  Hesse. 


GLOSSARY.  229 

Malignite,  a  name  proposed  by  Lawson  for  a  group  of  rocks  on  the 
Maligne  river,  Rainy  Lake  district,  province  of  Ontario.  They  are 
described  as  "basic,  holocrystalline,  plutonic  rocks,  rich  in  alkalies  and 
lime."  Iron  is  present  in  moderate  amounts,  almost  entirely  combined 
in  the  silicates.  Iron  and  magnesia  are  more  abundant  than  is  usual  in 
the  alkali-rich  plutonic  rocks.  The  chief  minerals  are  orthoclase,  often 
microscopically  intergrown  with  an  acid  plagioclase;  aegirite-augite, 
which  may  predominate  with  but  a  moderate  admixture  of  biotite,  or 
may  be  subordinate  and  intergrown  with  preponderant  soda  amphibole, 
biotite,  being  present  as  before.  There  are  three  types  of  malignites, 
one  of  which  has  much  melanite  and  another  much  nephelite.  Bull. 
Dept.  Geol.  Univ.  Calif.,  I.,  340,  1896. 

Manganolite,  Wadsworth's  name  for  rocks  composed  of  manganese 
minerals,  such  as  wad,  psilomelane,  etc.  Kept.  State  Geol.  Mich., 
1891-92,  p.  93. 

Mangerite,  a  name  based  upon  Manger,  a  Norwegian  locality,  by  C. 
F.  Kolderup  and  applied  to  granitoid  rocks  consisting  essentially  of 
microperthite  and  augite.  By  dynamic  metamorphism  the  augite  may 
pass  into  hornblende  and  biotite.  Gneissoid  structures  are  also  in- 
duced. Quartz-mangerites  represent  acid  facies.  The  rocks  are 
associates  of  the  anorthosites  of  Norway.  (Die  Labradorfelsen  des 
westlichen  Norwegens,  Bergens  Museums  Aarbog,  1903,  102.)  The 
same  rocks  are  abundant  in  the  Adirondacks  where  they  are  commonly 
called  syenites. 

Marble,  in  lithology,  a  metamorphosed  and  recrystallized  limestone. 
In  the  trade  the  name  is  applied  to  any  limestone  that  will  take  a 
polish. 

Marekanite,  a  rhyolitic  perlite  from  the  banks  of  the  Marekana  river, 
near  Ochotsk,  Siberia.  At  times  a  very  clear  glass,  it  is  found  in  balls 
and  cores  of  large  perlitic  masses  and  may  even  be  under  strain  like 
Prince  Rupert's  drops.  See  Zirkel's  Petrographie,  II.,  299. 

Mariupolite,  a  name  derived  from  Mariupol,  a  locality  on  the  sea  of 
Azov,  and  applied  by  J.  Morozewicz  to  a  variety  of  nephelite-syenite, 
so  rich  in  soda  and  poor  in  potash  that  orthoclase  practically  fails.  An 
estimate  of  the  percentages  of  the  component  minerals  gave,  albite,  73, 
nephelite,  14,  aegirite,  7.6,  lepidomelane,  4,  zircon,  1.6.  The  texture 
varies  from  coarsely  crystalline  to  porphyritic  and  to  compact,  according 
to  the  occurrence  of  the  rock  in  large  masses  or  in  dikes.  Tsch.  Mitth., 
XXL,  241,  1902.  Compare  litchfieldite. 

Markfieldite,  a  hypabyssal  igneous  rock,  intermediate  between 
granophyre  and  dolerite;  i.  e.,  a  dioritic  granophyre.  (Hatch,  Text- 
book of  Petrology,  5th  ed.,  219,  1909.) 


230  A  HAND  BOOK  OF  ROCKS. 

Marl,  a  calcareous  clay,  or  intimate  mixture  of  clay  and  particles  of 
calcite  or  dolomite,  usually  fragments  of  shells.  Marl  in  America  is 
chiefly  applied  to  incoherent  sands,  but  abroad  compact,  impure  lime- 
stones are  also  called  marls. 

Marmorosis,  the  general  name  for  the  process  of  crystallization  of 
limestones  to  marble,  whether  by  contact  or  regional  metamorphism. 
It  was  coined  by  Geikie  from  the  Latin  for  marble. 

Marscoite,  an  intermediate  contact  rock,  produced  by  the  action  of 
granite  during  deep-seated  stages,  upon  included  fragments  of  gabbro. 
Various  new  minerals  result  and  old  ones  have  new  physical  properties. 
(A.  Harker,  Tertiary  Igneous  Rocks  of  Skye,  Mem.  Geol.  Survey, 
United  Kingdom,  1904,  175,  192.) 

Massif,  a  single  mountainous  mass,  which  may  be  considered  a  unit. 

Massive,  the  antithesis  of  stratified,  and  therefore,  often  used  as  a 
synonym  of  igneous  or  eruptive  rocks  as  contrasted  with  the  bedded 
sedimentary  and  laminated  metamorphic  varieties. 

Mediophyric,  moderately  porphyritic;  phenocrysts  1-5  mm.  If 
spelled  mediiphyric  the  term  is  used  only  in  a  microscopic  sense; 
phenocrysts,  .O4~.oo8  mm.  Jour,  of  Geol.,  14:  702,  1906. 

Megascopic,  a  descriptive  term  meaning  large  enough  to  be  distin- 
guished with  the  naked  eye;  the  antithesis  of  microscopic.  See  mac- 
roscopic. Used  also  to  describe  methods  of  observation  without  the 
microscope  or  with  the  eye  alone. 

Megaphenocrysts,  large  phenocrysts.  (Jour.  Geol.,  14:  702,  1906.) 
The  texture  is  megaphyric. 

Melanocratic,  a  name  applied  by  W.  C.  Brogger  to  those  eruptive 
rocks,  in  which  the  dark  or  ferromagnesium  minerals  are  in  excess  over 
the  light  ones.  The  antithetical  term  is  leucocratic.  Melanocratic  is 
derived  from  two  Greek  words  meaning  the  black  prevails." 

Melaphyre,  a  rock  name  first  introduced  by  Brongniart  in  1813,  practi- 
cally for  porphyritic  rocks  with  a  dark  groundmass  and  with  feldspar 
phenocrysts.  After  having  had  various  meanings  for  many  years,  by 
common  consent,  it  is  now  generally  used  as  suggested  by  Rosenbusch 
for  pretertiary  olivine-basalts,  that  is,  for  porphyritic  equivalents  of 
olivine-diabase. 

Melilite,  the  name  of  the  mineral  is  sometimes  prefixed  to  the  names 
of  rocks  containing  it,  as  melilite-monchiquite. 

Melilite-basalt,  a  rare  basaltic  rock  whose  feldspathoid  is  melilite. 
It  was  first  identified  by  Stelzner  in  1882.  The  rock  is  excessively  basic. 
Alnoite  is  the  same  rock  in  dikes. 

Mesostasis,  a  synonym  of  basis  suggested  by  Giimbel. 


GLOSSARY.  231 

Meta,  the  Greek  preposition  for  after,  used  as  a  prefix  in  the  names 
of  various  rocks  which  are  derived  from  preexisting  rocks;  or  in  the 
names  of  various  geologic  processes  involving  change. 

Metabolite,  Wadsworth's  name  for  altered,  glassy  trachytes,  of  which 
lassenite  is  the  unaltered  form.  Kept.  State  Geol.  Mich.,  1891-92,  p.  97. 

Metachemical  metamorphism,  Dana's  term  to  describe  that  variety 
of  metamorphism  that  involves  a  chemical  change  in  the  rocks  affected. 
Amer.  Jour.  Sci.,  July,  1886,  p.  69. 

Metacrystal,  see  under  Brotocrystal. 

Metadiabase,  a  shortened  form  of  metamorphic  diabase,  suggested 
by  Dana  for  certain  rocks  simulating  diabase,  but  supposed  to  have 
been  produced  by  the  metamorphism  of  sediments.  Amer.  Jour.  Sci., 
Feb.,  1876,  121.  Compare  Pseudo-diabase. 

Metadiorite,  dioritic  rocks  produced  as  just  described  under  meta- 
diabase.  Compare  Pseudo-diorite. 

Metagabbro,  a  metamorphosed  gabbro:  necessarily  therefore  a  gabbro 
more  or  less  completely  changed  to  an  amphibolite.  (Amer.  Jour.  Sci., 
Feb.,  1904,  145.) 

Metarhyolite,  a  name  applied  to  rocks  which  were  originally  rhyo- 
lite,  but  which  are  now  altered  in  mineralogy,  by  recrystallization, 
so  as  to  develop  microperthite,  or  products  of  devitrification,  or  some 
other  change  from  their  original  condition.  Bull  U.  S.  Geol.  Survey, 
No.  150,  p.  164.  Aporhyolite  being  generally  accepted,  metarhyolite 
would  appear  to  be  superfluous. 

Metamorphism,  a  collective  term  for  the  process  by  which  rocks 
undergo  alteration  of  all  sorts.  It  is  more  fully  set  forth  on  page  122. 

Metasomatic,  i.  e.,  a  change  of  substance;  it  is  used  to  describe  the 
replacement  of  one  or  more  of  the  minerals  of  a  rock  by  others.  The 
form  of  the  originals  is  not  at  all  preserved  as  in  pseudomorphs,  nor 
does  the  chemical  composition  remain  the  same  while  the  form  alters  as 
in  paramorphs,  but  both  customarily  change.  The  term  is  especially 
used  in  connection  with  the  origin  of  ore  deposits.  The  corresponding 
noun  is  metasomatosis,  but  replacement  is  a  good  English  equivalent. 

Metastable,  the  condition  which  is  reached  by  a  cooling  solution, 
when,  being  supersaturated,  crystallization  is  instantly  induced  by  the 
introduction  of  a  solid,  though  perhaps  very  minute.  (See  ladings, 
Igneous  Rocks,  i:  159,  1910.) 

Metaxite,  a  name  of  Hauy's  for  micaceous  sandstone. 

Mexican  Onyx,  banded,  stalagmitic  marble,  originally  from  Mexico 
and  used  as  an  ornamental  stone.  The  name  is  now  generally  em- 
ployed as  a  trade  designation. 


232  A  HAND  BOOK  OF  ROCKS. 

Mezo  or  Meso  is  sometimes  prefixed  to  the  names  of  igneous  rocks  of 
Mesozoic  age. 

Miarolitic,  a  descriptive  term  applied  to  those  granitoid  rocks  that 
have  small  cavities,  into  which  well-terminated  crystals  project. 
See  p.  17. 

Miascite,  a  name  coined  from  Miask,  a  locality  in  the  Urals  where  a 
nephelite-syenite  occurs  whose  dark  silicate  is  biotite.  Used  also  as  a 
general  name  for  biotitic  nephelite-syenites.  See  p.  52. 

Mica,  the  name  of  the  mineral  is  often  prefixed  to  the  name  of  the 
rock  containing  it,  as,  mica-basalt,  mica-tinguaite,  mica-trachyte,  etc. 

Mica-peridotite,  a  name  applied  by  J.  S.  Diller  to  a  peculiar  perido- 
tite,  occurring  as  a  dike  in  Crittenden  County,  Ky.,  and  consisting 
chiefly  of  altered  olivine  and  biotite.  Amer.  Jour.  Sci.,  Oct.,  1892, 
288.  See  Analysis  8,  p.  81. 

Mica-schist,  finely  laminated,  metamorphic  rocks,  consisting  of 
quartz,  mica,  feldspar  and  several  minor  minerals.  See  p.  137. 

Mica-syenite,  syenite  whose  dark  silicate  is  biotite. 

Mica-trap,  an  English  field  name  for  dark,  dike  rocks  rich  in  mica. 

Microdiabase,  a  name  given  by  Lessen  to  aphanitic  diabases. 

Microdiorite,  a  name  originally  given  by  Lepsius  to  a  fine-grained 
diorite-porphyry,  Das  westliche  Siid-Tirol,  1878. 

Micro-felsite,  a  name  used  in  microscopic  work  for  those  varieties  of 
groundmass  that  do  not  affect  polarized  light,  but  that  are  not  true 
glasses  because  they  have  a  fibrous,  a  granular  or  some  such  texture. 
The  textures  are  no  doubt  in  many  cases  the  results  of  the  devitrifica- 
tion of  a  glassy  base. 

Micro-granite,  a  name  used  in  microscopic  work  for  those  ground- 
masses  of  porphyritic  rocks,  that  consist  of  small  quartz  and  feldspar 
crystals  with  granitoid  texture  on  a  small  scale,  i.  e.,  with  components 
of  about  the  same  size  and  usually  without  crystallographic  boundaries. 
See  granophyric. 

Micro-granulite,  the  French  equivalent  of  granophyric,  as  earlier 
explained. 

Micro-crystalline,  granular  rocks,  whose  components  are  recogni- 
zable, but  are  so  small  as  to  require  the  microscope  for  their  identification. 

Microlites,  generally  used  for  microscopic,  but  still  identifiable 
minerals. 

Micropegmatite,  i.  e.,  microscopic  pegmatite,  a  term  applied  to  those 
groundmasses  of  porphyritic  rocks  whose  microscopic  quartz  and  feld- 
spars mutually  penetrate  each  other.  The  several  parts  of  the  same 
crystal,  though  isolated,  extinguish  together.  See  granophyric. 


GLOSSARY.  233 

Microperthite,  i.  e.,  microscopic  perthite,  a  term  applied  to  that 
variety  of  orthoclase  which  is  thickly  set  with  flat  spindles  of  albite.  It 
is  very  common  in  gneisses.  Compare  granophyric. 

Microphenocrysts,  phenocrysts  only  observed  with  the  microscope. 
The  texture  is  microphyric.  (Jour.  Geol.,  14:  702,  1906.) 

Micropoikilitic,  a  textural  term  suggested  by  G.  H.  Williams  to  de- 
scribe those  minerals  that  are  speckled  with  microscopic  inclusions  of 
other  minerals,  having  no  definite  relations  to  each  other  or  to  their 
host.  Jour,  of  Geology,  I.,  176,  1893.  Poikilitic  is  often  spelled  poicil- 
itic  or  poecilitic. 

Mijakite,  an  andesite  from  the  Japanese  island  of  Mijakeshima  from 
which  the  name  is  derived.  It  is  porphyritic  with  phenocrysts  of  by- 
townite,  augite,  hypersthene  and  biotite.  In  the  groundmass  are  brown 
pyroxene,  feldspar  and  basis.  Largely  on  the  results  of  the  chemical 
analysis,  the  brown  pyroxene  is  believed  to  be  a  manganese-bearing, 
triclinic  variety  related  to  babingtonite,  hence  the  new  name  for  the 
rock.  J.  Petersen,  Jb.  Hamburg  Welt  Ausstellung,  VIII.,  50,  1891. 

Millstone-grit,  an  old  English  name  for  the  conglomeratic  sandstone 
at  the  base  of  the  Carboniferous  Coal  Measures.  It  is  more  or  less  cur- 
rent in  this  country  as  a  synonym  of  the  Great,  Pottsville  or  Serai  con- 
glomerate. 

Mimesite,  an  obsolete  synonym  of  dolerite. 

Mimophyre,  a  name  suggested  by  Elie  de  Beaumont  in  1841  for  meta- 
morphosed, argillaceous  rocks  in  which  feldspars  had  developed,  so 
that  they  resembled  porphyries.  Volcanic  tufts  are  a  frequent  original, 
but  graywackes  and  arkoses  have  also  yielded  them.  Compare  Por- 
phyroid. 

Mineralizers,  the  dissolved  vapors  in  an  igneous  magma,  such  as 
steam,  hydrofluoric  acid,  boracic  acid  and  others,  that  exert  a  powerful 
influence  in  the  development  of  some  minerals  and  textures.  See  p.  1 8. 
The  word  is  also  technically  used  in  some  definitions  of  ore.  Thus  it  is 
said  that  an  ore  is  a  compound  of  a  metal,  and  a  mineralizer  which 
disguises  its  metallic  properties;  such  as  copper  with  sulphur,  iron  with 
oxygen,  etc. 

Minette,  a  variety  of  mica-syenite,  usually  dark  and  fine-grained, 
occurring  in  dikes.  See  p.  44,  Anal.  6. 

Minophyric,  minutely  porphyritic,  phenocrysts  1-0.2  mm.  Mini- 
phyric  is  used  in  a  microscopic  sense,  phenocrysts,  less  than  .008  mm. 
(Jour.  Geol.,  14:  702,  1906.) 

Missourite,  a  granitoid  rock  consisting  of  leucite,  biotite,  augite, 
olivine,  iron  ores  and  apatite,  and  corresponding  to  the  effusive  leucite- 


234  A  HAND  BOOK  OF  ROCKS. 

basalts.  It  was  discovered  in  the  Highwood  Mountains,  Mont.,  by 
Weed  and  Pirsson,  and  named  by  them  from  the  Missouri  River,  ''the 
most  prominent  and  best  known  geographical  object  in  the  region." 

Moldauite,  a  very  pure  glass,  from  the  valley  of  the  Moldau  river, 
Bohemia,  and  believed  to  be  meteoritic  in  origin.  See  Bouteillenstein. 

Monchiquite,  a  name  suggested  by  Hunter  and  Rosenbusch  from  the 
Monchique  Mountains  of  Portugal,  for  basaltic  dikes  corresponding  in 
mineralogy  and  texture  to  limburgite.  They  often  accompany  nephe- 
lite-syenite.  Tsch.  Mitt.,  XL,  445,  1890.  In  modification  of  the 
original  view  that  the  monchiquites  have  a  glassy  groundmass,  L.  V. 
Pirsson  has  urged  with  much  reason  and  with  the  additional  evidence  of 
chemical  analysis,  that  the  supposed  glass  is  analcite.  The  presence 
of  so  much  glass  in  so  basic  a  rock  is  improbable.  Journal  of  Geology, 
IV.,  679. 

Mondhaldeite,  a  name  derived  from  a  locality  on  the  Kaiserstuhl, 
Baden,  and  applied  by  A.  Osann  to  a  group  of  dike  rocks  having  the 
mineralogy  of  the  hornblende-pyroxene  andesites.  Chemically  they  are 
andesites  of  about  60  per  cent,  in  silica,  and  with  almost  as  much  potash 
as  soda.  Tscherm.  Mitth.,  XXL,  416,  1902. 

Monmouthite,  a  basic,  granitoid  rock  whose  essential  minerals  are 
nephelite  and  hornblende,  and  whose  more  frequent  accessories  are 
plagioclase,  cancrinite,  and  calcite,  with  sodalite,  apatite,  sphene,  bio- 
tite,  pyrite  and  iron  ores  in  extremely  small  amount.  The  monmouthite 
appears  as  bands  produced  by  magmatic  differentiation  in  an  albitic 
nephelite  syenite  (litchfieldite)  along  its  contact  with  limestone.  On 
analysis  the  chief  constituents  of  monmouthite  were  the  following:  SiO2 
39.74,  A12O,  30.59,  FeO  2.19,  CaO  5-75,  K2O  3.88,  Na2O  13.25,  CO2 
2.17,  H2O  i.oo;  all  the  rest  1.29,  no  one  being  over  .60,  total  99.86. 
The  name  was  given  by  F.  D.  Adams  and  is  derived  from  the  township 
of  Monmouth,  Ontario.  (Amer.  Jour.  Sci.,  April,  1904,  269.) 

Monzonite  has  usually  been  considered  as  a  variety  of  augite-syenite 
that  displayed,  however,  considerable  mineralogical  variety.  Brogger 
has  recently  used  the  name  for  a  transitional  and  intermediate  group  of 
granitoid  rocks  between  the  granite-syenite  series  (i.  e.,  the  alkali-feld- 
spar series)  and  the  diorites  (i.  e.,  the  lime-soda  feldspar  series).  The 
monzonites  have  both  alkali-feldspar  (or  orthoclase)  and  lime-soda  feld- 
spar (or  plagioclase)  in  approximately  equal  amounts,  or  at  least  both 
richly.  (Die  Eruptivgesteine  des  Kristianiagebietes,  II.,  21,  1895.) 
With  the  meaning  proposed  by  Brogger  the  name  has  been  much  used 
by  the  U.  S.  Geol.  Survey  in  monographs  on  western  mining  districts. 
If  some,  but  not  much  quartz  is  present  the  rock  becomes  quartz-mon- 


GLOSSARY.  235 

zonite,  as  in  the  basic  granite  at  Butte,  Mont.  Increasing  quartz  leads 
to  grano-diorite. 

Mortar-structure,  a  term  suggested  by  Tornebohm  to  describe  those 
granites,  gneises  or  other  rocks  that  have  been  dynamically  crushed,  so 
that  large  nuclei  of  their  original  minerals  are  set  in  crushed  and  com- 
minuted borders  of  the  same,  like  stones  in  a  wall. 

Mugearite,  the  hypabyssal  equivalent  of  essexite.  It  consists  of 
oligoclase,  subordinate  orthoclase,  olivine,  augite,  apatite  and  iron  ore. 
(Alfred  Harker,  Mem.  Geol.  Surv.  of  Great  Britain,  1904,  265.) 

Mulatto,  a  local  name  in  Ireland  for  a  Cretaceous  green  sand. 

Muscovado,  the  Spanish  word  for  brown  sugar,  used  by  Minnesota 
geologists  for  a  rusty,  brown,  outcropping  rock  that  resembles  brown 
sugar.  It  has  been  applied  to  both  gabbros  and  quartzites.  i6th 
Ann.  Kept.  Minn.  Geol.  Surv. 

Mylonite,  a  name  suggested  by  the  English  geologist  Lapworth  for 
schists  produced  by  dynamic  metamorphism.  Rept.  of  Brit.  Assoc., 
1885-86,  p.  1025. 

Myrmekite,  graphic  granite,  the  eutectic  of  quartz  and  feldspar, 
common  in  pegmatites. 

N 

Nadel-diorite,  i.  e.,  needle-diorite,  a  German  term  for  diorites  with 
acicular  hornblende. 

Napoleonite,  a  synonym  of  corsite. 

Natron-granite,  granites  abnormally  high  in  soda,  presumably  from 
the  presence  of  an  orthoclase  rich  in  soda,  or  of  anorthoclase.  They 
are  also  called  soda-granites.  Natron  is  likewise  used  as  a  prefix  to 
minerals  and  rocks  that  are  rich  in  soda,  as  natron-orthoclase,  natron- 
syenite,  etc. 

Navite,  Rosenbusch's  name  for  pretertiary,  porphyritic  rocks,  con- 
sisting of  plagioclase,  augite  and  olivine  as  phenocrysts,  with  a  second 
generation  of  the  same  forming  the  holocrystalline  groundmass.  The 
name  is  from  Nava,  a  locality  in  the  Nahe  Valley.  Mass.  Gest.,  1887. 

Necks,  lava-filled  conduits  of  extinct  volcanoes,  exposed  by  erosion. 

Nelsonite,  a  name  derived  from  Nelson  Co.  Virginia,  and  given  by 
T.  L.  Watson  to  a  rock  consisting  of  rutile  and  apatite  as  essential 
minerals,  with  ilmenite,  pyriteand  quartz  as  accessories.  It  is  mined 
as  a  source  of  titanium  and  is  obviously  related  to  the  pegmatites. 
Compare  Krageroite.  (Mineral  Resources  of  Virginia,  300,  1907.) 

Neolite,  a  name  used  by  Clarence  King  for  an  order  of  volcanic 
rocks,  embracing  the  rhyolites  and  basalts,  with  which  according  to  the 


236  A  HAND  BOOK  OF  ROCKS. 

succession  formulated  by  von  Richthofen,  eruptive  activity  terminates 
in  any  given  area.  Geol.  Survey  of  the  Fortieth  Parallel,  I.,  689. 

Nepheline,  see  Nephelite. 

Nephelinite,  basaltic  rocks  consisting  of  nephelite,  augite  and  basis, 
but  without  olivine.  See  p.  72. 

Nephelinitoid,  Boricky's  term,  now  used  in  microscopic  work  for 
nepheline-glass,  or  the  glassy  basis  in  nepheline  rocks,  whose  easy  gela- 
tinization  indicates  its  close  relations  with  this  mineral ;  unindividualized 
nephelite. 

Nephelite,  a  later  method  of  spelling  nepheline  and  one  consistent 
with  approved  mineralogical  orthography. 

Nephelite-basalt,  an  old,  general  name  for  basaltic  rocks  with  nephe- 
lite, but  now  restricted  to  those  that  practically  lack  plagioclase,  and 
that  have  nephelite,  augite,  olivine  and  basis.  See  p.  72. 

Nephelite-basanite,  basaltic  rocks  with  plagioclase,  nephelite,  augite, 
olivine  and  basis.  Compare  nephelite-tephrite.  See  p.  72. 

Nephelite  syenite,  *'.  e.,  eleolite-syenite,  a  name  to  be  preferred  to 
the  latter  as  there  is  no  real  need  of  the  word  eleolite.  Granitoid  rocks 
consisting  of  orthoclase,  nephelite,  and  one  or  more  of  the  following: 
hornblende,  augite  and  biotite.  The  rocks  result  from  magmas  espe- 
cially rich  in  alkalies,  and  possess  great  scientific  interest  on  account  of 
their  richness  in  rare,  associated  minerals.  See  p.  51. 

Nevadite,  a  name  coined  by  von  Richthofen  from  Nevada,  for  those 
rhyolites  that  approximate  a  granitoid  texture,  *.  e.,  with  little  ground- 
mass.  Mem.  Calif.  Acad.  Sci.,  I.,  p.  54,  1867.  See  p.  24  and  Hague 
and  Iddings,  Amer.  Jour.  Sci.,  June,  1884,  461. 

NeVe,  a  French  synonym  of  firn. 

Nonesite,  porphyrites  with  orthorhombic  pyroxene.  The  name  was 
given  by  Lepsius.  Das  westliche  Sad-Tyrol,  Berlin,  1878. 

Nordmarkite,  Brogger's  name  for  a  variety  of  granitic  rocks  consisting 
of  orthoclase,  some  oligoclase,  more  or  less  microperthite,  quartz  and 
somewhat  subordinate  biotite,  pyroxene,  hornblende  and  aegirite. 
Nordmarkites  are  high  in  silica  and  the  alkalies.  Zeitsch.  f.  Kryst., 
XVI.,  54,  1890. 

Norite,  a  rock  of  the  gabbro  family  that  consists  of  plagioclase  and 
orthorhombic  pyroxene,  usually  hypersthene.  The  name  has  had  a 
variable  history  and  was  originally  proposed  in  1832  by  Esmark  for 
aggregates  of  feldspar  and  hornblende  which  were  lacking  or  notably 
poor  in  diallage  and  hypersthene.  But  as  many  localities  were  cited 
which  in  later  years  on  microscopic  examination  were  found  rich  in  these 
minerals,  Rosenbusch  finally  gave  the  name  its  above  definition  and  this 
is  its  generally  accepted  signification. 


GLOSSARY.  237 

Normal  metamorphism,  *'.  e.,  regional  metamorphism.     See  p.  123. 

Normal-pyroxenic,  Bunsen's  name  for  his  assumed,  typical,  basic,  ig- 
neous magma  with  48  per  cent.  SiO2  as  contrasted  with  the  correspond- 
ing normal-trachytic  one  with  76  per  cent.  SiC>2.  He  sought  to  explain 
all  intermediate  rocks  by  the  intermingling  of  these  two.  Although  ap- 
parently applicable  at  times  and  serviceable  in  their  day,  the  concep- 
tions have  long  since  been  exploded.  See  J.  Roth's  Gesteinsanalysen, 
1861. 

Nosean,  the  name  of  the  mineral  is  often  prefixed  to  the  names  of 
rocks  containing  it. 

Novaculite,  excessively  fine  grained,  quartzose  rocks  supposed  to  be 
consolidated,  siliceous  slimes  and  of  sedimentary  origin.  They  are 
especially  developed  in  Arkansas,  and  are  much  used  as  whetstones. 
See  pp.  97,  98. 

o 

Obsidian,  a  general  name  for  volcanic  glass.  When  used  alone  it 
implies  a  rhyolite-glass,  but  it  is  now  much  employed  with  a  prefix  as 
andesite-obsidian,  basalt-obsidian.  See  p.  26. 

Ocellar-structure,  a  microscopic  term  used  by  Rosenbusch  for  pe- 
culiar aggregates  of  small  pyroxenes,  that  resemble  eyes,  buds  and  the 
like,  and  that  are  especially  common  in  nepheline  and  leucite  rocks. 
Mass.  Gest.,  625,  1887. 

Odinite,  a  name  given  by  Chelius  to  certain  porphyritic  dikes  in  Mt. 
Melibocus,  which  have  a  groundmass  of  plagioclase  and  hornblende  rods, 
with  phenocrysts  of  plagioclase  and  augite.  Notizbl.  Ver.  Erdkunde, 
Darmstadt,  1892,  Heft  13,  p.  i. 

Oikocryst,  the  containing  crystal  of  the  inclusions  in  the  poikilitic 
texture.  (Jour.  Geol.,  14:  704,  1906.)  The  inclusions  are  xenocrysts. 
The  abundance  of  xenocrysts  indicated  by  the  terms  peroikic,  domoikic, 
xenoikic,  doxenic,  perxenic — ranging  from  few  to  many.  (Jour.  Geol., 
14:  704,  1906.) 

Olivine,  the  name  of  the  mineral  is  prefixed  to  the  names  of  many 
rocks  that  contain  it.  Olivine  is  of  especial  importance  in  this  respect 
as  its  presence  marks  a  more  basic  development  in  the  rocks,  which  have 
it  as  contrasted  with  those  which  lack  it. 

Oolitic,  a  textural  term  for  those  rocks  which  consist  of  small  concre- 
tions, analogous  to  the  roe  of  fish.  Oolites  are  calcareous,  siliceous  and 
ferruginous. 

Opacite,  a  noncommittal  microscopic  term,  less  current  than  formerly, 
for  minute,  opaque  grains  observed  in  thin  sections  of  rocks.  They  are 
generally  regarded  to-day  as  chiefly  magnetite  dust. 


238  A  HAND  BOOK  OF  ROCKS. 

Ophicalcite,  Brongniart's  name  for  crystalline  limestones,  spotted 
with  serpentine.  See  p.  152. 

Ophiolite,  Brongniart's  name  for  the  serpentines.  See  p.  152.  It  is 
also  employed  in  America  in  the  sense  of  ophicalcite  as  above  given. 

Ophite,  a  name  given  in  1798  by  the  Abbe  Palassou  to  a  green  rock 
of  the  Pyrenees.  It  was  later  recognized  to  be  composed  of  feldspar  and 
hornblende,  and  still  later  was  determined  by  Zirkel  to  be  uralitized 
diabase.  The  name  has  chief  significance  to-day  because  used  to  de- 
scribe the  textural  peculiarity  of  some  diabases.  Strictly  speaking  an 
ophitic  texture  is  one  in  which  rod-like  or  lath-shaped,  automorphic 
plagioclase  feldspars  are  involved  in  augite,  as  it  were,  in  a  paste,  so  as 
to  form  a  variety  of  poicilitic  texture,  but  the  term  was  used  in  the  first 
edition  of  this  book,  and  is  employed  by  many  in  the  sense  in  which 
the  diabasic  texture  is  defined  on  a  preceding  page.  The  difference 
between  the  two  meanings  lies  in  the  fact  that  in  the  former  the  augite 
is  in  excess,  and  the  feldspar  is  involved  in  it.  In  the  latter,  the  feld- 
spar is  in  excess,  and  the  augite  fills  the  interstices  between  its  lath- 
shaped  crystals.  The  peculiar  significance  of  these  textures  is  that  the 
feldspars  crystallized  before  the  augite,  contrary  to  the  usual  succession. 
See  p.  77  and  p.  78. 

Orbicular,  a  textural  term  for  those  rare  rocks  whose  minerals  have  a 
spheroidal  grouping,  such  as  corsite  and  orbicular  granite.  See  Kugel 
and  Spheroidal. 

Orbite,  a  name  proposed  by  Chelius  for  certain  diorite  dikes  near 
Orbeshohe,  Hesse,  of  porphyritic  texture  and  having  large  phenocrysts 
of  hornblende,  biotite  and  plagioclase.  Notizbl.  Ver.  Erd.  Darmstadt, 
1892,  i. 

Orendite,  a  name  proposed  by  Whitman  Cross,  for  the  peculiar  leu- 
citic  rocks  at  Orenda  Butte  in  the  Leucite  Hills,  Wyo.  They  contain 
leucite  and  sanidine,  in  about  equal  amounts,  with  phlogopite  and 
diopside  as  essentials.  A  peculiar  amphibole  is  also  present.  The  rock 
is  a  leucite-phonolite  as  the  latter  term  is  used  by  older  writers,  but 
the  objection  to  calling  any  rock  a  phonolite  which  lacks  nephelite,  led 
to  the  name  orendite,  Amer.  Jour.  Sci.,  Aug.,  1897,  p.  123.  Com- 
pare Madupite  and  Wyomingite,  etc. 

Orio-crystal,  see  Brotocrystal. 

Ornoite,  a  dioritic  rock  from  the  Swedish  locality  of  Ornd.  It  con- 
tains prevailing  oligoclase,  with  some  hornblende  and  very  subordinate 
microcline  and  orthoclase.  The  accessories  are  apatite,  magnetite, 
pyrite,  titanite  and  a  little  prehnite.  The  name  was  given  by  A.  Ceder- 
strom.  Geol.  Foren.  Forhand.,  XV.,  108,  1893. 


GLOSSARY.  239 

Orthoclase,  the  name  of  the  mineral  is  often  prefixed  to  the  names  of 
rocks  that  contain  it. 

Orthofelsite,  a  name  suggested  by  J.  J.  H.  Teall,  for  porphyritic 
rocks  with  felsitic  groundmass,  and  phenocrysts  of  orthoclase.  British 
Petrography,  291,  1888. 

Orthogneiss,  Rosenbusch's  name  for  micaceous  gneisses  derived  from 
igneous  rocks.  (Elemente  der  Gesteinslehre,  484,  1901.)  The  con- 
trasted term  is  paragneiss. 

Orthophyre,  i.  e.,  orthoclase  porphyry  or  porphyry  proper. 

Ortlerite,  a  name  given  by  the  Austrian  geologists,  Stache  and  von 
John,  to  certain  porphyrites  of  the  eastern  Alps  that  resemble  the  old 
greenstones  and  that  have  plagioclase,  hornblende,  generally  augite, 
and  more  or  less  basis.  The  range  from  48-54  SiC>2.  Jahrb.  k.  k.  g. 
Reichsanst.,  1870,  342. 

Ossipyte,  a  name  suggested  by  C.  H.  Hitchcock  for  a  rock  from 
Waterville,  N.  H.,  which  on  examination  in  1871  by  E.  S.  Dana  (be- 
fore the  use  of  thin  sections  in  America)  was  thought  to  consist  of  oli- 
vine  and  labradorite,  with  a  little  magnetite.  Ossipyte  is  derived  from 
"Ossipees,"  the  name  of  a  tribe  of  Indians,  who  formerly  lived  in  the 
region.  Amer.  Jour.  Sci.,  Jan.,  1872,  p.  49.  By  means  of  thin  sections 
the  rock  was  later  shown  to  contain  diallage,  by  G.  W.  Hawes,  and  to  be 
a  gabbro.  Geol.  of  New  Hampshire,  Vol.  III.,  Part  IV.,  p.  166.  Os- 
sipyte was  a  forerunner  of  troctolite  over  which  it  has  priority. 
Additional  notes  on  the  local  geology  will  be  found  in  the  paper  by  L. 
V.  Pirsson  and  W.  N.  Rice.  Amer.  Jour.  Sci.,  Apr.,  1911,  269-291, 
especially  279.  The  modern  spelling  is  ossipite. 

Ottrelite  schists,  schistose  rocks  with  the  peculiar  micaceous  mineral 
ottrelite.  They  are  best  known  from  the  Ardennes,  Belgium,  but  are 
found  in  New  England. 

Ouachltite  (pronounced  waw-shee-tite),  a  name  coined  by  Kemp  from 
the  Ouachita  River,  Arkansas,  for  certain,  basic  dikes  containing,  in  a 
glassy  groundmass,  prevailing  and  often  phenomenally  large  phenocrysts 
of  biotite,  very  subordinate  augite  and  magnetite.  They  also  occur  at 
Beemerville,  N.  J.,  associated  with  nephelite  syenite.  Ann.  Rep. 
Geol.  Surv.  of  Ark.,  1890,  II.,  393. 

Oiyphyre,  Pirsson's  general  name  for  the  acidic  rocks.  Oxyphyre 
is  contrasted  with  Lamprophyre,  a  corresponding  name  for  the  basic 
rocks.  The  two  are  complementary,  see  Lamprophyre,  and  Comple- 
mentary Rocks. 


240  A  HAND  BOOK  OF  ROCKS. 

P 

Pahoehoe,  the  Hawaiian  word  for  a  lava  sheet,  whose  surface  consists 
of  smooth  or  fluted  hummocks.  It  is  contrasted  with  aa,  which  refers 
to  jagged  and  cindery  crusts.  See  Aa.  It  has  been  specially  introduced 
into  English  speech  by  Capt.  (now  Major)  C.  E.  Button.  4th  Ann. 
Rep.  Dir.  U.  S.  Geol.  Surv.,  95,  1883. 

Paisanite,  a  name  proposed  by  Osann  from  the  Paisano  Pass,  on  the 
Southern  Pacific  R.  R.,  in  western  Texas,  for  a  variety  of  quartz-por- 
phyry, consisting  of  phenocrysts  of  microperthitic  orthoclase  and  rarer 
quartz,  in  a  groundmass  of  quartz  and  feldspar.  Occasional  groups  of 
small  hornblendes  (riebeckite)  appear.  Tscherm.  Min.  u.  Petr.  Mitth., 
XV.,  435.  Compare  Comendite. 

Palaeophyre,  Gumbel's  name  given  in  1874,  to  certain  porphyritic 
dike  rocks  corresponding  to  quartz-mica-diorites  in  mineralogy.  They 
cut  the  Silurian  strata  ot  the  Fichtelgebirge. 

Palaeophyrite,  a  name  proposed  by  Stache  and  von  John  (compare 
ortlerite)  for  certain  porphyrites  in  whose  strongly  prevailing  ground- 
mass  are  phenocrysts  of  plagioclase,  hornblende  and  augite.  Jahrb.  d. 
k.  k.  g.  Reichsanstalt,  1879,  342. 

Palasopicrite,  a  name  proposed  by  Gumbel  in  1874.  in  his  paper, 
"Die  palaeolithischen  Eruptiv-gesteine  des  Fichtelgebirges,"  for  picrites 
which  were  considered  by  him  to  be  similar  to  the  rocks  from  the  Cre- 
taceous formation,  originally  named  picrite  by  Tschermak.  Gumbel 
called  his  specimens  pal seo picrites  because  they  occurred  in  Palaeozoic 
strata.  He  regarded  them  as  aggregates  of  olivine,  enstatite,  chrome- 
diopside  and  magnetite,  but  they  are  now  known  to  be  chiefly  olivine 
and  augite.  More  or  less  brown  hornblende  and  biotite  also  occur. 

Palagonite-tuff,  an  altered  basaltic  tuff  from  Palagonia,  in  Sicily. 
The  name  palagonite  was  originally  applied  to  problematical,  brown 
inclusions  in  the  tuff,  which  were  thought  at  first  to  be  a  definite  min- 
eral. They  are  now  known  to  be  a  devitrified,  basaltic  glass.  The 
name  was  given  by  v.  Waltershausen  in  1846.  See  Vulk.  Gesteine  in 
Sicilien  und  Island,  1853,  179. 

Palatinite,  a  name  proposed  by  Laspeyres  for  certain  rocks  in  the 
German  province  of  Pfalz,  supposed  by  him  to  be  gabbros  with  diallage 
and  to  be  of  Carboniferous  age;  but  they  have  since  been  shown  to  be 
essentially  diabases.  Neues  Jahrb.,  1869,  516.  The  word  is  derived 
from  the  classic  name  of  the  district. 

Pallasite,  originally  proposed  by  Gustav  Rose  for  a  meteorite  that 
fell  near  Pallas,  in  Russia,  has  been  used  by  Wadsworth  in  a  wider  sense 
for  both  meteoric  and  terrestrial,  ultra-basic  rocks,  which  in  the  former 


GLOSSARY.  241 

average  about  60  per  cent,  iron,  and  in  the  latter  have  at  least  more 
iron  oxides  than  silica.  Cumberlandite  (which  see)  is  the  chief  exam- 
ple. Lithological  Studies,  1884,  68. 

Panidiomorphic,  Rosenbusch's  term  for  those  rocks,  all  of  whose  com- 
ponents possess  their  own  crystal  boundaries.  Panautomorphic  is  a 
synonym. 

Pantellerite,  a  group  of  rocks  intermediate  between  the  rhyolites  and 
trachytes  on  the  one  hand,  and  the  dacites  on  the  other.  They  differ 
from  all  these  in  having  anorthoclase  as  the  principal  feldspar.  Cossy- 
rite,  a  rare  and  probably  titaniferous  amphibole,  occurs  at  the  original 
locality  on  the  island  of  Pantelleria,  in  the  Mediterranean.  See  pp.  31, 
43,  58.  The  name  was  given  by  Forstner.  Zeitschr.  f.  Kryst.,  1881, 
348. 

Paragenesis,  a  general  term  for  the  order  of  formation  of  associated 
minerals  in  time  succession,  one  after  the  other.  To  study  the  para- 
genesis  is  to  trace  out  in  a  rock  or  vein  the  succession  in  which  the 
minerals  have  developed  and  if  some  are  secondary  after  others  the 
study  brings  out  these  relations. 

Paragneiss,  Rosenbusch's  name  for  micaceous  gneisses  derived  from 
sediments.  (Elemente  der  Gesteinslehre,  484,  1901.)  The  contrasted 
term  is  orthogneiss. 

Paramorphism,  the  passage  of  one  mineral  into  another  without 
change  of  composition,  as  augite  into  hornblende  in  uralitization.  It  is 
also  used  in  connection  with  metamorphism  to  describe  such  thorough 
changes  in  a  rock,  that  its  old  components  are  destroyed  and  new  ones 
are  built  up. 

Parophite,  a  name  given  by  T.  Sterry  Hunt,  Geol.  Surv.  Can.,  1852, 
95,  to  a  rock  or  mineral  similar  to  dysyntribite.  The  name  means 
like  serpentine. 

Patic,  a  word  derived  from  the  French  "pSte"  or  groundmass  and 
used  in  the  Quantitative  System  with  various  prefixes  to  describe  the 
several  porphyritic  textures.  Thus  "perpatic, "  extremely  rich  in 
groundmass;  "dopatic, "  groundmass  dominant.  Compare  semic. 
(Jour.Geol.,  14:  701,  1906.) 

Pearl-diabase,  see  Variolite. 

Pearlite  or  Pearlstone,  see  perlite. 

Pegmatite,  originally  applied  to  graphic  granite,  but  of  later  years 
used  as  a  general  name  for  very  coarse,  dike  or  vein  granites,  such  as 
have  large  quartz,  feldspar,  muscovite,  biotite,  tourmaline,  beryl  and 
other  characteristic  minerals,  and  are  often  called  giant-granite.  See 

P-  35- 
16 


242  A  HAND  BOOK  OF  ROCKS. 

Pole's  Hair,  a  fibrous,  basaltic  glass  from  the  Hawaiian  Islands, 
named  after  a  local  goddess. 

Pelite,  a  general  name  for  mud  rocks,  i.  e.,  shales,  clays  and  the  like. 

Pencatite,  see  predazzite. 

Per,  a  syllable  used  as  a  prefix  in  the  Quantitative  System  to  denote 
extreme  richness  in  any  mineral  or  character.  It  is  the  grade  beyond 
dominant,  as  perfemic,  persalic,  etc. 

Peridotite,  granitoid  rocks  consisting  of  olivine  and  pyroxene  with 
little  or  no  feldspar.  See  p.  81'  Many  varieties  has  been  made  de- 
pending on  the  kind  of  pyroxene  present,  or  on  its  absence  in  favor  of 
related  minerals,  viz: 

Olivine,  augite — Picrite. 

Olivine,  diopside  (diallage).  enstatite — Lherzolite. 

Olivine,  enstatite — Saxonite,  harzburgite. 

Olivine,  enstatite,  augite — Buchnerite. 

Olivine,  augite,  garnet — Eulysite  (metamorphic?). 

Olivine,  diallage,  hornblende — Wehrlite. 

Olivine,  hornblende — Cortlandtite. 

Olivine,  biotite — Mica-peridotite. 
,     Olivine,  hornblende  (secondary?),  biotite — Scyelite. 

Olivine,  alone  or  with  chromite — dunite. 

Further  particulars  about  each  of  these  will  be  found  under  the  indi- 
vidual names.     Compare  also  kimberlite. 

Perknite,  a  name  from  the  Greek  word  for  dark,  and  proposed  by  H. 
W.  Turner  as  a  collective  term  for  the  rocks  usually  called  pyroxenites 
and  hornblendites.  Mineralogically  the  perknites  consist  briefly  of 
monoclinic  pyroxene  and  amphibole  with  subordinate  orthorhombic 
pyroxene,  olivine  and  feldspar.  Chemically  they  are  lower  in  alumina 
and  alkalies  than  the  diorites  and  gabbros,  and  lower  in  magnesia  than 
the  peridotites.  Jour,  of  Geol.,  IX.,  507,  1901. 

Perlite,  volcanic  glass  and  concentric,  shelly  texture  and  usually  with 
a  notable  percentage  of  water.  See  p.  26. 

Perthite,  a  name  given  by  Thomson  Phil.  Mag.,  1843,  188)  to 
parallel  intergrowths  of  orthoclase  and  albite,  originally  described  from 
Perth,  Ontario. 

Petrical,  L.  Fletcher's  name  for  the  coarser  structural  features  of 
rocks.  See  lithical. 

Petrogenesis,  that  branch  of  geology  which  deals  with  the  origin  of 
rocks. 

Petrographic  Province,  an  area  whose  igneous  rocks,  of  whatever  type, 
are  characterized  by  some  distinctive  feature,  such  as  richness  in  soda, 
or  potash  or  lime.  Thus  Alfred  Harker  has  concluded  that  the  igneous 
rocks  around  the  Atlantic  Ocean  are  relatively  rich  in  the  alkalies; 


GLOSSARY.  243 

while  those  bordering  the  Pacific  are  relatively  rich  in  lime.  He  speaks 
of  the  Atlantic  province  and  the  Pacific  province  in  consequence.  In  a 
more  restricted  sense  the  region  just  east  of  or  just  within  the  Rocky 
Mountains  has  a  long  succession  of  rich  soda  rocks — phonolites,  etc., 
in  Montana,  Colorado,  Texas  and  Tamaulipas.  See  p.  91. 

Petrography,  properly  the  descriptive  part  of  the  science  of  rocks 
for  which  the  more  general  name  is  petrology  or  lithology,  but  petrog- 
raphy is  widely  used  as  a  synonym  of  the  latter. 

Petrosilex,  an  old  name  for  extremely  fine,  crystalline  porphyries 
and  quartz-porphyries  and  for  those  finely  crystalline  aggregates  we  now 
know  to  be  devitrified  glasses;  also  for  the  ground  masses  of  the  former, 
which  though  not  glassy  are  yet  not  resolvable  by  the  microscope  into 
definite  minerals.  See  felsite,  micro-felsite.  It  was  practically  a  confes- 
sion by  the  older  petrographers,  that  they  did  not  know  of  what  the  rock 
consisted. 

Phacolite,  a  mass  of  intrusive  rock  which  enters  the  crest  of  an  anti- 
cline or  the  trough  of  a  syncline  and  assumes  a  shape  like  a  lentil  or  half 
a  dried  pea.  In  igneous  rocks  the  term  is  analogous  to  "saddle  reefs" 
in  ore-deposits  and  is  derived  from  the  Greek  words  for  lentil  and  rock. 
A.  Marker,  Nat.  Hist.  Ign.  Rocks,  77,  1909. 

Phaneric,  textures  in  igneous  rocks  which  are  evident  to  the  unaided 
eye.  Its  contrasted  term  is  aphanitic.  (Jour.  Geol.,  14:  693,  1906.) 

Phanerohyaline,  obviously  glassy,  igneous  rocks.  (Jour.  Geol.,  14: 
693,  1906.) 

Phenocrysts,  a  name  suggested  by  J.  P.  Iddings  (Bull.  Phil.  Soc. 
Wash.,  XL,  73,  1889),  for  porphyritic  crystals  in  rocks.  It  has  proved 
an  extremely  convenient  one,  although  its  etymology  has  been  criticized. 
It  may  be  best  to  change  to  phanerocryst,  just  as  in  botanical  usage, 
phenogam  has  yielded  to  phanerogam;  but  one  form  or  the  other  is  a 
necessity. 

Phonolite,  volcanic  rocks,  of  porphyritic  or  felsitic  texture,  consist- 
ing of  orthoclase,  nephelite,  pyroxene  and  more  rarely,  amphibole. 
Leucite  may  replace  the  nephelite  and  yield  leucite-phonolites.  See  p. 
46.  The  name  is  Klaproth's  rendering  into  a  Greek  derivative  of  the 
old  name  Clinkstone.  Abhandl.  Berlin.  Akad.,  1801. 

Phosphorite,  massive  calcic  phosphate,  of  the  composition  of  apatite 
but  usually  lacking  crystal  form. 

Phosphorolite,  Wadsworth's  name  for  phosphatic  rocks,  guano-phos- 
phorite, apatite,  etc.  Rept.  State  Geol.  Mich.,  1891-92,  p.  93. 

Phthanite,  Hauy's  name  for  silicious  schists.  Its  use  has  recently 
been  revived  in  America  by  G.  F.  Becker,  who  applies  it  to  certain 


244  A  HAND  BOOK  OF  ROCKS. 

silicified  shales  in  California.  Quicksilver  deposits  of  the  Pacific  coast, 
Mono.  XIII.,  105,  U.  S.  Geol.  Survey. 

Phyllite,  a  name  for  intermediate  rocks  between  the  mica  schists  and 
slates,  usually  finely  crystalline;  mica-slates.  See  p.  139. 

Phyre,  a  modification  of  the  last  syllable  of  porphyry,  often  used 
with  prefixes,  as  vitrophyre,  orthophyre,  granophyre,  etc. 

Picrite,  a  name  originally  given  by  Tschermak  to  certain  porphyritic 
rocks  from  the  Carpathians,  that  have  abundant  and  large  phenocrysts 
of  olivine,  with  less  pyroxene,  hornblende  and  biotite,  in  a  glassy 
groundmass,  more  or  less  divitrified.  The  rocks  are  practically  pre-ter- 
tiary  limburgites.  Picrite  is  now  also  applied  to  those  peridotites  that 
consist  of  olivine  and  augite.  It  is  derived  from  the  Greek  for  bitter, 
in  allusion  to  the  high  percentage  of  magnesia,  Bittererde  in  German. 

Pilandite,  a  porphyritic  phase  of  hatherlite. 

Pistazite,  a  synonym  of  epidote,  more  current  in  Europe  than 
America,  and  used  in  rock  names  for  epidote. 

Pitchstone,  a  glassy  rock,  usually  corresponding  to  the  rhyolites  or 
trachytes,  but  with  a  considerable  percentage  of  water,  5-8  per  cent, 
for  example.  It  was  formerly  specially  used  for  pretertiary  glasses,  «'.  e., 
the  glasses  of  quartz-porphyries  and  porphyries,  but  time  distinctions 
are  obsolete.  Pitchstones  have  a  marked  resinous  luster  as  the  name 
implies.  See  p.  26. 

Plagiaplite  an  aplitic  dike  rock,  consisting  of  acidic  plagioclase, 
quartz  and  a  little  hornblende.  (L.  Duparc  and  S.  Jerchoff,  Arch.  Sci. 
phys.  et  nat.  Geneva,  Feb.,  1902,  cited  by  Rosenbusch,  Mikr.  Phys., 
4th  ed.,  II.,  590.) 

Planophyric,  having  the  phenocrysts  of  a  porphyritic  rock,  arranged 
in  layers.  (Jour.  Geol.,  14:  703,  1906.) 

Plumasite,  a  dike-rock  consisting  essentially  of  oligoclase  and  corun- 
dum. It  cuts  peridotite  near  the  Diadem  mine,  Plumas  Co.,  California, 
and  was  first  described  and  named  by  A.  C.  Lawson,  Bull.  Dept.  Geol- 
ogy, Univ.  of  Calif.,  III.,  219,  1903. 

Plutonic,  a  general  name  for  those  rocks  that  have  crystallized  in  the 
depths  of  the  earth,  and  have  therefore  assumed  as  a  rule,  the  granitoid 
texture.  See  p.  16 

Pneumatolitic,  a  general  name  for  those  minerals  which  have  been 
produced  in  connection  with  igneous  rocks  through  the  agency  of  the 
gases  or  vapors  called  mineralizers.  They  may  be  in  the  igneous  mass 
itself  or  in  cracks  in  the  wall  rock.  Compare  the  cases  cited  on  pp. 
125,  130.  The  term  is  much  used  in  discussions  of  ore  deposits. 

Poicilitic,  i.  e.,  speckled,  a  term  proposed  by  G.  H.  Williams  for 


GLOSSARY.  245 

those  rocks  which  have  mottled  luster,  because  on  the  shining  cleavage 
faces  of  some  of  their  minerals,  small  inclusions  of  others  occur,  pro- 
ducing the  effect.  The  same  thing  was  earlier  called  "luster  mottling" 
by  Pumpelly,  but  poicilitic  has  proved  a  useful  term  both  in  megascopic 
and  microscopic  work.  (Journal  of  Geology,  I.,  176,  1893.)  It  is 
also  spelled  poikilitic  and  poecilitic. 

Porcellanite,  fused  shales  and  clay,  that  occur  in  the  roof  and  floor 
of  burned  coal  seams.  The  rock  is  quite  common  in  the  lignite  dis- 
tricts of  the  West,  where  apparently  spontaneous  combustion  has  fired 
the  seams  in  the  past. 

Porodine,  Breithaupt's  name  for  amorphous  rocks,  such  as  are  derived 
from  gelatinous  silica. 

Porodite,  Wadsworth's  name  proposed  in  1879,  for  all  the  altered, 
fragmental  forms  of  eruptive  rocks,  commonly  called  diabase  tuff,  schal- 
stein,  etc.  Bull.  Mus.  Comp.  Zool.,  1879,  V.,  280. 

Porphyrite,  a  porphyritic  rock,  belonging  to  the  plagioclase  series 
and  corresponding  in  mineralogy  to  the  diorites.  To  distinguish  it 
from  andesite,  it  is  necessary  to  draw  a  contrast  between  surface  flows 
(andesites)  and  intruded  dikes  or  sheets  (porphyrites) ;  or  between  ter- 
tiary and  later  lavas  (andesites)  and  pretertiary  ones  (porphyrites) ;  or 
between  those  with  glassy  or  very  finely  crystalline  groundmasses  (ande- 
sites) and  those  with  groundmasses  of  moderate  coarseness  (porphyrites). 

Porphyritic,  a  textural  term  for  those  rocks  which  have  larger  crys- 
tals (phenocrysts)  set  in  a  finer  groundmass,  which  may  be  crystalline 
or  glassy,  or  both.  See  p.  1 6.  Rosenbusch  has  sought  to  define  it  as 
the  texture  due  to  the  recurrence  of  the  period  of  crystallization  of  the 
same  or  similar  minerals  (Neues  Jahrb.,  1882,  II.,  3).  While,  except 
for  porphyritic  rocks  with  a  glassy  groundmass,  this  practically  amounts 
to  the  same  thing  as  the  textural  definition  just  given,  it  is  idle  for  any 
writer  to  try  to  change  so  old,  well-established  and  indispensable 
a  conception. 

Porphyry,  a  word  derived  from  the  classic  name  of  the  mollusc,  a 
species  of  Murex,  that  yielded  the  Tyrian  purple  of  the  ancients.  It 
was  later  applied  to  the  red,  porphyritic  rock  of  the  Egyptian  quarries, 
"  porfido  rosso  antico,"  whose  red  color  is  due  to  piedmontite,  a  man- 
ganese epidote.  In  course  of  time  it  was  applied  to  all  porphyritic 
rocks  as  we  now  understand  the  term.  In  its  restricted  sense  it  implies 
orthoclase-porphyry,  the  porphyritic  rock  corresponding  to  syenite,  but 
to  give  it  any  essential  significance  as  contrasted  with  trachyte,  one  of 
the  three  distinctions  must  be  drawn,  which  are  cited  above  under  por- 
phyrites, and  of  which  the  second  is  of  no  real  value.  See  p.  43.  For- 


246  A  HAND  BOOK  OF  ROCKS. 

phyry  is  colloquially  used  for  almost  every  igneous  rock  in  the  West, 
that  occurs  in  sheets  or  dikes  in  connection  with  ore  bodies. 

Porphyroid,  metamorphic  rocks  with  porphyritic  texture,  i.  e.,  with 
phenocrysts  of  feldspar  or  other  minerals  in  a  finer  groundmass,  yet 
shown  by  geological  relations  to  be  altered  sediments,  or  tuffs.  Fossil 
remains  have  even  been  detected  in  some.  They  are  close  relatives  of 
halleflintas. 

Pozzuolane,  a  leucitic  tuff,  found  near  Naples  and  used  for  hydraulic 
cement. 

Predazzite,  a  contact  rock  at  Predazzo  in  the  Tyrol,  produced  from 
crystalline  dolomite  by  an  intrusion  of  syenite.  It  is  partly  calcite  and 
partly  brucite  or  hydromagnesite.  Pencatite  is  the  same  aggregate, 
darkened  by  grains  of  pyrrhotite. 

Primary,  an  old  synonym  of  Archean.  The  word  is  also  used  for 
those  rocks  which  have  crystallized  directly  from  fusion  or  solution,  as 
contrasted  with  transported  or  secondary  sediments;  and  for  the  min- 
erals of  an  igneous  or  metamorphic  rock,  which,  originating  in  situ,  date 
from  the  crystallization  of  the  rock  itself;  as  contrasted  with  secondary 
minerals  produced  in  alteration,  or  weathering.  See  p.  12. 

Prismoid,  textures  in  igneous  rocks  whose  components  are  prismatic. 
They  may  be  parallelopipedons,  lath-shaped  blades,  prisms,  spindles  or 
fibers.  (Jour.  Geol.,  14:  699,  1906.) 

Propylite,  a  name  given  by  von  Richthofen  in  1867  to  certain  ande- 
sites,  formed  at  the  beginning  of  Tertiary  time,  that  were  thought  to  re- 
isemble  the  old  diorites,  and  diorite-porphyrites.  They  had  been  previ- 
ously called  by  him  greenstone-trachytes  in  Hungary,  but  were  not  named 
propylite  until  he  met  them  again  in  Nevada  and  California  (Memoirs 
of  the  California  Academy  of  Sciences,  I.,  60,  1867).  The  western 
propylites  have  been  since  conclusively  shown  by  several  American 
petrographers  to  be  only  more  or  less  altered  andesites.  The  literature 
of  the  name  furnishes  an  interesting  and  amusing  exhibition  of  the  efforts 
of  those  petrographers,  who  were  influenced  by  the  time-myth  in  the 
classification  of  igneous  rocks,  to  draw  distinctions,  where  there  were  no 
differences.  The  name  means  before  the  gates,  alluding  to  their  position 
at  the  beginning  or  entrance  to  the  Tertiary,  which  was  supposed  to 
usher  in  the  true,  volcanic  eruptions  of  geological  time. 

Proteolite,  an  old  name  for  certain  contact  rocks  produced  by  granite 
intrusions  from  slates  in  Cornwall.  It  has  been  lately  revived  by  Bonny 
for  andalusite-hornfels.  (Quar.  Jour.  Geol.  Soc.,  1886,  104.)  Com- 
pare Cornubianite. 

Proterobase,  originally  applied  by  Gumbel   1874,  to  Silurian  or  ear- 


GLOSSARY.  247 

Her  diabases  with  hornblende.  The  frequency  of  the  paramorphism  of 
augite  to  hornblende  has  led  others  to  apply  it  to  diabases  with  uralitized 
augite.  Rosenbusch  restricts  it  to  diabases  with  original  hornblende. 
Protogine,  an  old  name  for  a  granite  or  gneiss  in  the  Alps,  consisting 
of  quartz,  orthoclase  and  chlorite  or  sericite,  the  last-named  of  which 
was  formerly  erroneously  taken  for  talc.  The  laminated  structure  from 
dynamic  metamorphism  is  often  pronounced. 

Psammites,  a  general  name  for  sandstones,  from  the  Greek  word  for 
a  grain  of  sand. 

Psephites,  a  general  name  for  conglomerates  and  breccias,  i.  e., 
coarse,  fragmental  rocks  as  contrasted  with  psammites  and  pelites.  The 
name  is  derived  from  the  Greek  for  pebble. 

Pseudo-diabase,  a  name  proposed  by  G.  F.  Becker  for  certain  meta- 
morphic  rocks  in  the  Coast  ranges  of  California  that  are  supposed  to 
have  been  derived  from  sediments,  yet  that  have  the  minerals  and  tex- 
ture of  diabase.  Monograph  XIII.,  U.  S.  Geol.  Surv.,  p.  94.  Com- 
pare Metadiabase,  which  means  the  same  thing  and  has  precedence. 
Pseudo-diorite,  dioritic  rocks  produced  as  described  under  pseudodia- 
base  above.  See  the  same  reference. 

Pseudo-chrysolite,  synonym  of  moldauite,  bouteillenstein. 
Pseudomorph,  the  replacement  of  one  mineral  by  another,  such  that 
the  form  of  the  first  is  preserved  by  the  second,  despite  the  difference  in 
composition. 

Puddingstone,  conglomerate. 

Pulaskite,  a  special  name  given  by  J.  Francis  Williams  to  certain 
syenitic  rocks  from  Pulaski  County,  Arkansas,  that  have  trachytic  tex- 
ture and  that  consist  of  orthoclase  (kryptoperthite),  hornblende  (arfved- 
sonite),  biotite  and  a  little  augite  (diopside),  eleolite,  sodalite  and  ac- 
cessory minerals.  Ann.  Rep.  Geol.  Surv.  Ark.,  1890,  II.,  56.  Com- 
pare laurvikite. 

Pumice,  excessively  cellular,  glassy  lava,  generally  of  the  composi- 
tion of  rhyolite.  See  p.  26. 

Pyroclastic,  fragmental  rocks  such  as  tuffs  and  breccias,  produced 
by  explosive,  igneous  action. 

Pyrogenic,  a  general  name  proposed  by  A.  W.  Grabau  for  the  igneous 
rocks,  i.  e.,  those  which  originate  through  the  agency  of  heat.  The 
name  makes  one  in  a  logical  group,  pyrogenic;  hydrogenic;  atmogenic. 
Pyromeride,  a  name  given  by  the  Abbe  Hauy  to  the  orbicular  diorite 
or  corsite  of  Corsica.  The  word  means  "partly  fusible,"  and  refers  to 
the  properties  of  the  two  constituent  minerals,  of  which  the  one,  quartz, 
was  infusible,  and  the  other,  the  feldspar,  could  be  melted. 


248  A  HAND  BOOK  OF  ROCKS. 

Pyrosehists,  a  name  suggested  by  T.  Sterry  Hunt  for  those  sediments 
that  are  impregnated  with  combustible,  bituminous  matter.  Amer. 
Jour.,  March,  1863,  159 

Pyroxene,  the  name  of  the  mineral  is  often  prefixed  to  the  name 
of  the  rocks  that  contain  it. 

Pyroxenite,  a  name  first  proposed  by  T.  Sterry  Hunt  for  the  masses 
of  pyroxene  occurring  with  the  apatite  deposits  of  Canada.  It  is  now 
generally  employed  in  the  sense  advocated  by  G.  H.  Williams,  for 
granitoid,  non-feldspathic  rocks,  whose  chief  mineral  is  pyroxene,  and 
which  lack  olivine.  See  p.  81.  (Amer.  Geologist,  July,  1890,  p.  47.) 
Williams  proposed  the  name  websterite,  from  Webster,  N.  C.,  for  a 
variety  consisting  of  diopside  and  bronzite,  with  the  latter  porphyriti- 
cally  developed.  Idem,  35.  Compare  perknite. 

Q 

Quantitative  System,  a  system  for  the  classification  of  the  igneous 
rocks,  proposed  in  its  completed  form  in  1903  by  Whitman  Cross,  J.  P. 
Iddings,  L.  V.  Pirsson  and  H.  S.  Washington.  It  is  primarily  based  on 
the  quantitative  relations  of  the  rock-making  minerals  in  the  igneous 
rocks  as  their  relative  percentages  by  weight  may  be  calculated,  meas- 
ured or  otherwise  determined.  The  usual  method  is  to  recast  a  chemical 
analysis  by  the  methods  explained  on  earlier  pages,  but  when,  from 
lack  of  data,  this  proves  impossible,  so-called  standard  minerals  are  used 
which  are  not  necessarily  and  are  seldom  actually  in  the  rock.  The 
system  has  proved  of  great  value  in  bringing  our  relationships  among 
rocks  of  which  satisfactory  chemical  analyses  have  been  prepared,  and 
is  specially  adapted  to  more  advanced  methods  of  research.  (See 
Quantitative  Classification  of  Igneous  Rocks,  Chicago,  1903.  G.  I. 
Finlay,  Calculation  of  the  Norm  in  Igneous  Rocks.  Jour.  Geology,  18: 
58-92,  1910.) 

Quartz,  the  name  of  the  mineral  is  prefixed  to  the  names  of  many 
rocks  that  contain  it,  as  quartz-porphyry,  p.  31;  quartz-diorite,  p.  59, 
etc. 

Quartz-diorite,  an  igneous  rock,  of  a  granitoid  texture,  chief  feldspar 
plagioclase,  essential  quartz,  usually  some  orthoclase;  dark  silicates 
biotite,  hornblende  or  augite,  one  or  several;  deep-seated  equivalent 
of  the  dacites;  closely  related  to  grano-diorites  and  quartz-monzonites. 

Quartzite,  metamorphosed  sandstone.  See  p.  144.  Not  to  be  used 
for  vein  quartz. 

Quartz-monzonite,  igneous  rocks,  of  granitoid  texture,  of  mineralogy 
intermediate  between  syenites  and  diorites  (see  monzonite)  and  with 
some  quartz,  but  not  enough  to  make  them  grano-diorites. 


GLOSSARY.  249 

Quartz-porphyry,  an  igneous  rock  of  porphyritic  texture;  chief 
phenocrysts  quartz  and  orthoclase;  usually  some  plagioclase;  sub- 
ordinate biotite,  hornblende  or  augite,  one  or  several;  groundmass 
felsitic  or  finely  crystalline,  rarely  glassy.  The  name  was  formerly 
restricted  to  pre-tertiary  rocks,  but  this  usage  is  obsolete.  The  name 
has  been  used  for  intrusive  dikes  and  sheets  as  contrasted  with  the 
rhyolites,  which  are  characteristic  surface  flows.  In  the  previous 
pages  it  is  replaced  by  rhyolite-porphyry,  but  it  is  still  and  for  many 
years  will  remain  a  widely  used  field-name. 

R 

Radial  Dikes,  a  descriptive  term  specially  used  by  L.  V.  Pirsson  for 
those  dikes  which  radiate  outward  from  an  eruptive  center.  Amer. 
Jour.  Sci.,  Aug.,  1895,  p.  116. 

Reaction-rims,  a  term  mostly  used  in  microscopic  work,  for  the 
curious  rims  of  hypersthene,  garnet,  hornblende,  biotite,  magnetite  and 
perhaps  other  minerals,  that  surround  grains  of  magnetite  or  of  ferro- 
magnesian  silicates,  wherever,  in  many  gabbros,  they  would  other- 
wise come  next  to  feldspar.  They  are  supposed  to  be  produced  by 
the  reaction  of  these  latter  minerals  on  each  other,  probably  in  the 
crystallization  of  the  rock.  A  synonym  is  corona-structure.  (See  J. 
F.  Kemp,  Bull.  Geol.  Soc.  of  Amer.,  V.,  221,  1894.) 

Regional-metamorphism,  Daubree's  name  for  that  extended  meta- 
morphism  that,  as  contrasted  with  contact  effects,  is  manifested  over 
large  areas.  See  pp.  131-133. 

Regolith,  a  name  coined  by  G.  P.  Merrill  from  two  Greek  words 
meaning  "blanket  of  stone"  for  the  layer  of  loose  materials  that  mantles 
the  land  areas  of  the  globe  and  rests  on  the  solid  rocks.  These  mate- 
rials are  derived  from  the  decay  of  rocks,  accumulations  of  vegetation, 
talus,  debris,  sediments,  wind-blown  sands  and  glacial  deposits.  Rocks, 
Rock-weathering  and  Soils,  299,  1897. 

Rensselaerite,  E.  Emmons'  name  for  a  talcose  rock  from  St.  Law- 
rence Co.,  N.  Y.  Annual  report  of  the  N.  Y.  Geol.  Survey,  1837,  p. 
152. 

Resorbed,  a  term  used  in  microscopic  work  to  describe  those  pheno- 
crysts which  after  crystallization  are  partly  fused  again  into  the  magma. 
See  p.  21. 

Retinite,  the  current  name  for  pitchstone  among  the  French. 

Rhomben-porphyries,  a  name  applied  to  certain  Norwegian  por- 
phyries, whose  phenocrysts  of  orthoclase  are  bounded  by  oo  P  and  2P  ~a°, 
so  that  they  resemble  a  rhombohedron.  The  orthoclase  is  rich  in  soda. 


250  A  HAND  BOOK  OF  ROCKS. 

Rhyocrystal,  see  Brotocrystal. 

Rhyolite,  volcanic  rocks  of  porphyritic  or  felsitic  texture,  whose 
phenocrysts  are  prevailingly  orthoclase  and  quartz,  less  abundantly 
biotite,  hornblende  or  pyroxene,  and  whose  groundmass  is  crystalline, 
glassy,  or  both.  The  name  is  from  the  Greek  to  flow,  and  refers  to  the 
frequent  flow  structure.  Rhyolite  is  current  in  America,  whereas  liparite 
and  quartz-trachyte  are  more  used  abroad.  The  name  was  given  in 
i860  by  v.  Richthofen.  (Jahrb.  d.  k.  k.  Reichsanst.,  XL,  153,  1860.) 

Rill-marks,  small  depressions  in  sandstones,  produced  by  the  eddying 
of  a  retreating  wave  on  a  seabeach  under  the  lee  of  some  small  obstruc- 
tion, such  as  a  shell  or  pebble. 

Ripple-marks,  corrugations  in  sandstones  produced  by  the  agitation 
of  waves  or  winds,  when  the  rock  was  being  deposited. 

Rizzonite,  a  name  derived  from  a  locality,  in  the  Monzoni  region,  and 
suggested  by  Doelter  and  Went  for  a  limburgite  dike.  (Sitzungsber. 
Wiener  Akad.,  Jan.  15,  1903.) 

Rockallite,  a  name  proposed  by  J.  W.  Judd  for  a  rock  from  Rockall 
Island,  a  small  reef  in  the  North  Atlantic,  240  miles  west  of  Ireland. 
Rockallite  is  a  granitoid  rock,  consisting  of  quartz,  albite  and  a^girite, 
in  proportions  respectively  of  38  :  23  :  39,  in  the  specimen  investigated. 
Trans.  Royal  Irish  Academy,  XXXI. ,  Part  III.,  39;  Amer.  Jour.  Sci., 
March,  1899,  241. 

Rock-flour,  a  general  name  for  very  finely  pulverized  rocks  or  min- 
erals which  lack  kaolinite  and,  therefore,  the  plasticity  of  clay,  and  which 
are  much  finer  than  sand.  Rock-flour  which  is  largely  pulverized 
quartz,  may  be  separated  from  most  clays. 

Routivarite,  a  name  derived  from  the  famous  Swedish  locality  Routi- 
vara,  where  titaniferous  iron  ores,  with  abundant  spinel,  occur.  It 
was  applied  by  H.  Sjogren  to  a  phase  of  rock  bordering  on  the  ore  and 
consisting  of  striated  and  unstriated  feldspar,  quartz  and  game  (Geol. 
For.  i.  Stockholm.  Forh.,  XV.,  62,  1893.) 

Rudite,  a  general  name  suggested  by  A.  W.  Grabau  for  very  coarse 
sediments,  analogous  in  size  of  components  to  rubble-masonry,  which 
the  name  means.  Prefixes  such  as  calci-rudite  and  silico-rudite,  de- 
scribe the  components  of  the  aggregate.  Calci-rudite  has  been  applied 
with  special  significance  to  those  coarsely  fragmental  limestones  pro- 
duced by  the  breaking  down  of  coral  reefs.  (Bull.  Geol.  Soc.  Amer.,  14: 
348.) 

s 

Saccharoidal,  a  term  applied  to  sandstones  whose  texture  resembles 
that  of  old-fashioned  loaves  of  sugar. 


GLOSSARY.  251 

Sagvandite,  a  curious  rock  from  near  Lake  Sagvand,  Norway,  that  is 
mainly  bronzite  and  magnesite.  A  little  colorless  mica,  and  more  or 
less  chromite  and  pyrite  are  also  present.  The  name  was  given  by 
Petterson.  Neues  Jahrb.,  1883,  II.,  247. 

Sahlite,  a  variety  of  pyroxene,  sometimes  prefixed  to  rock  names. 

Salband,  a  term  current  among  miners  for  the  parts  of  a  vein  or  dike 
next  to  the  country  rock. 

Salfemic,  rocks  on  the  border  between  salic  and  femic.  See  salic 
below. 

Salic,  a  name  coined  from  silica-alumina  and  first  used  in  the  Quanti- 
tative System  for  those  igneous  rocks  with  predominant  light  colored 
minerals,  i.  e.,  quartz  and  feldspars  and  for  the  minerals  themselves. 
Its  antitheiss  is  femic.  Salic  and  femic  have  proved  very  useful  being 
shorter  and  more  practicable  than  leucocratic  and  melanocratic,  or  in 
the  case  of  femic,  than  ferromagnesian. 

Sand,  incoherent  fragments  of  minerals  or  rocks  of  moderate  size,  say 
one-quarter  of  an  inch  (6  mm.)  and  less  in  diameter.  Quartz  is  much 
the  commonest  mineral  present.  See  p.  97. 

Sandstone,  consolidated  sands.     See  p.  97. 

Sanidinite,  a  name  applied  especially  to  certain  trachytic  bombs  that 
occur  in  tuffs  in  the  extinct  volcanic  district  of  the  Laacher  See,  Germany. 
Recently  it  has  been  suggested  by  Weed  and  Pirsson  for  the  extreme 
case  of  feldspathic  syenites,  in  which  all  other  minerals  except  orthoclase 
practically  fail.  They  establish  a  series  as  follows: 

All  orthoclase,  no  augite — Sanidinite. 

Orthoclase  exceeds  augite — Augite-syenite. 

Orthoclase  equals  augite — Yogoite. 

Augite  exceeds  orthoclase — Shonkinite. 

All  augite,  no  orthoclase — Pyroxenites  of  various  types. 

Amer.  Jour.  Sci.,  Dec.,  1895,  p.  479.  Subsequently  yogoite  was 
withdrawn  in  favor  of  monzonite,  which  has  priority  but  which  is  now 
used  in  a  different  sense.  Idem,  May,  1896,  357,  358. 

Santorinite,  a  name  proposed  by  H.  S.  Washington  for  those  excep- 
tional andesitic  or  basaltic  rocks,  which,  with  a  high  percentage  of  silica 
(65-69),  yet  have  basic  plagioclases,  of  the  labradorite-anorthite  series. 
The  name  was  suggested  by  the  volcano  Santorini.  (Journal  of 
Geology,  V.,  May-June,  1897,  368.)  See  also  Fouqu6,  Santorini  et 
ses  Eruptions,  Paris,  1879,  and  Etude  des  Feldspaths,  317-320.  The 
prevailing  bisilicate  at  Santorini  is  pyroxene. 

Sanukite,  Weinschenk's  name  for  a  glassy  phase  of  andesite  that 
contains  bronzite,  augite,  magnetite,  and  a  few  large  plagioclases  and 


252  A  HAND  BOOK  OF  ROCKS. 

garnets.  The  rock  is  related  to  the  andesites  as  are  the  limburgites  to 
the  basalts.  Neues  Jahrb.  Beilageband,  VII.,  148,  1891. 

Saprolite,  a  name  suggested  by  G.  F.  Becker  for  those  superficial 
deposits  produced  by  the  decay  of  rocks  and  remaining  as  residuals. 
It  is  in  a  measure  synonymous  with  laterite.  Compare  also  regolith. 
Saprolite  is  from  the  Greek  for  ''  rotten  rock."  (i6th  Ann.  Rep.  U.  S. 
Geol.  Survey,  3:  289,  1895.) 

Saussurite-gabbro,  gabbro  whose  feldspar  is  altered  to  saussurite. 
See  p.  141. 

Sazonite,  Wadsworth's  name  for  peridotites  consisting  of  enstatite  or 
bronzite  and  olivine.  It  is  a  synonym  of  harzburgite,  but  saxonite  has 
priority.  Lithological  Studies,  1884,  p.  85. 

Schalstein,  an  old  name  for  a  metamorphosed  diabase  tuff. 

Schiller-fels,  enstatite  or  bronzite  peridotite  with  poicilitic  pyrox- 
enes. Orthorhombic  pyroxenes  possess  the  poicilitic  texture  to  a 
peculiar  degree,  and  especially  when  more  or  less  altered  to  bastite. 
The  term  schiller,  which  expresses  this,  is  especially  appropriate  for  them. 

Schillerisation,  Judd's  name  for  the  process  of  producing  poicilitic 
texture  by  the  development  of  inclusions  and  cavities  along  particular 
crystal  planes.  The  cavities  are  largely  produced  by  solution,  some- 
what as  are  etch  figures,  and  are  afterwards  filled  by  infiltration. 
Quart.  Jour.  Geol.  Soc.,  1885,  383;  1886,  82. 

Schist,  thinly  laminated,  metamorphic  rocks  which  split  more  or  less 
readily  along  certain  planes  approximately  parallel.  See  p.  137. 

Schlieren,  a  useful  German  term,  largely  adopted  into  English,  for 
those  smaller  portions  of  many  igneous  rocks,  which  are  strongly  con- 
trasted with  the  general  mass,  but  which  shade  insensibly  into  it.  Thus 
portions  of  granite  are  met,  much  richer  in  biotite  and  hornblende  than 
the  normal  rock,  or  much  more  coarsely  crystalline.  Pegmatite  streaks 
occur  and  other  differentiations  of  the  original  magma.  Several  differ- 
ent varieties  may  be  made,  for  a  discussion  of  which  see  Zirkel's  Lehr- 
buch  der  Petrographie,  I.,  787,  1893. 

Schorl,  an  old  name  for  tourmal  ne,  still  sometimes  used  in  names  of 
rocks. 

Schriesheimite,  a  dike-rock  of  the  composition  of  amphibole-perido- 
tite  and  having  a  marked  poikilitic  texture.  It  was  named  from  the 
Schriesheim  valley  near  Heidelberg.  (H.  Rosenbusch,  Mikros.  Phys., 
4th  ed.,  II.,  458.) 

Scoria,  coarse,  cellular  lava,  usually  of  basic  varieties. 

Scyelite,  Judd's  name  for  a  rock,  related  to  the  peridotites,  that  occurs 
near  Loch  Skye,  in  Scotland.  Its  principal  mineral  is  green  hornblende, 


GLOSSARY.  253 

presumably  secondary  after  augite;  with  it  are  bleached  biotites,  to- 
gether with  serpentine,  supposed  to  be  derived  from  olivine.  See  Quar. 
Jour.  Geol.  Soc.,  1885,  401. 

Secondary,  a  term  used  both  for  rocks  and  minerals,  that  are  derived 
from  other  rocks  and  minerals.  Examples  are  sandstone,  clay,  or  other 
sediments;  chlorite  from  augite,  etc.  See  the  contrasted  word  primary. 

Sedimentary,  rocks  whose  components  have  been  deposited  from 
suspension  in  water.  See  p.  92. 

Selagite,  a  name  of  Hauy's  for  a  rock  consisting  of  mica,  disseminated 
through  an  intimate  mixture  of  amphibole  and  feldspar,  but  it  has  been 
since  applied  to  so  many  different  rocks  as  to  be  valueless. 

Selenolite,  Wadsworth's  name  for  rocks  composed  of  gypsum  or 
anhydrite.  Kept.  State  Geol.  of  Mich.,  1891-92,  p.  93. 

Semic,  a  word  derived  from  the  French  "seme,"  meaning  sown  or 
sprinkled,  and  used  for  those  porphyritic  textures  which  are  sprinkled 
with  phenocrysts;  "sempatic,"  phenocrysts  and  groundmass  approxi- 
mately equal,  dosemic,  phenocrysts  dominate;  persemic,  extremely  rich 
in  phenocrysts.  (Jour.  Geol.,  14:  701,  1906.) 

Septaria,  literally  little  walls,  a  name  applied  to  concretions,  largely 
of  argillaceous  material,  which  are  traversed  by  cracks.  The  cracks  are 
filled  as  a  rule  with  calcite  or  quartz,  affording  an  intersecting  network 
from  which  weathering  may  have  removed  the  original,  included,  argil- 
laceous matter. 

Seriate,  inequigranular  textures  in  igneous  rocks,  whose  components 
vary  gradually  or  in  a  continuous  series.  (Jour.  Geol.,  14;  700:  1906.) 
When  the  sizes  are  abruptly  contrasted  or  in  broken  series  the  term  is 
hiatal. 

Sericite-schist,  mica-schist  whose  mica  is  sericite.  See  p.  139. 
Sericite  is  also  used  as  a  prefix  to  many  names  of  metamorphic  rocks 
containing  the  mineral. 

Serpentine,  a  metamorphic  rock  consisting  chiefly  of  the  mineral 
serpentine.  See  p.  150. 

Shastalite,  Wadsworth's  name  for  unaltered,  glassy  forms  of  andesite. 
Kept,  of  Mich.  State  Geol.,  1891-92,  p.  97. 

Shonkinite,  a  name  given  by  Weed  and  Pirsson  to  a  rock  from  the 
Highwood  Mountains,  Mont.,  which  they  define  as  "  a  granular,  plutonic 
rock  consisting  of  essential  augite  and  orthoclase,  and  thereby  related 
to  the  syenite  family.  It  may  be  with  or  without  olivine,  and  accessory 
nepheline,  sodalite,  etc.,  may  be  present  in  small  quantities."  Bull. 
Geol.  Soc.  Amer.,  VI.,  415,  1895.  See  Anal.  7,  p.  42.  Later  they  state 
that  augite  should  exceed  orthoclase.  Amer.  Jour.  Sci.,  Dec.,  1895, 
P-  479- 


254  A  HAND  BOOK  OF  ROCKS. 

Shoshonite,  a  general  name  proposed  by  Iddings  for  a  group  of  igne- 
ous rocks  in  the  eastern  portion  of  the  Yellowstone  Park.  They  are 
porphyritic  in  texture,  with  phenocrysts  of  labradorite,  augite  and 
olivine,  in  a  groundmass  that  is  glassy  or  crystalline;  in  the  latter  case 
orthoclase  and  leucite,  alone  or  together,  are  developed.  Chemically 
they  range:  SiO2,  50-56;  A12OS,  17-19.7;  CaO,  8-4.3;  MgO,  4.4-2.5; 
Na2O,  3-3.9;  K2O,  3.4-4.4.  The  rocks  are  to  be  considered  in  con- 
nection with  absarokite  and  banakite.  Jour,  of  Geol.,  III.,  937. 

Siderolite,  as  used  by  Fletcher  and  generally  in  English,  is  a  name 
for  meteorites  that  are  partly  metallic  iron  and  partly  silicates.  As 
used  by  others  it  is  applied  to  more  purely  metallic  ones. 

Sideromelane,  von  Waltershausen's  name  for  a  basaltic  glass  from 
the  palagonite  tuffs  of  Sicily.  Vulk.  Gest.  v.  Sicilien  und  Island,  202, 

1853- 

Silicalite,  Wadsworth's  name  for  rocks  composed  of  silica,  such  as 
diatomaceous  earth,  tripoli,  quartz,  lydite,  jasper,  etc.  Kept.  State 
Geol.  Mich.,  1891-92,  p.  92. 

Silicification,  the  entire  or  partial  replacement  of  rocks  and  fossils 
with  silica,  either  as  quartz,  chalcedony  or  opal. 

Sillite,  Giimbel's  name  for  a  rock  from  Sillberg,  in  the  Bavarian  Alps, 
variously  referred  by  others  to  gabbro,  diabase,  mica-syenite  and  mica- 
diorite.  Beschr.  der  bay.  Alpen,  184,  1861. 

Sills,  an  English  name  for  an  intruded  sheet  of  igneous  rock. 

Silt,  a  general  name  for  the  muddy  deposit  of  fine  sediment  in  bays 
or  harbors,  and  one  much  employed  in  connection  with  engineering 
enterprises. 

Sinaite,  an  alliterative  substitute  for  syenite  proposed  by  Rozieres 
because  on  Mt.  Sinai  true,  quartzless  syenites  occur,  whereas  at  Syene 
the  rock  is  a  hornblende-granite. 

Skarn,  a  term  current  among  the  Swedish  iron  miners  for  the  aggre- 
gates of  basic  silicates,  especially  hornblende,  biotite  and  pyroxene 
which  are  associated  with  the  magnetic  ores.  (Eleventh  Int.  Geol. 
Congr.  Guide-book.  Pamphlet  4,  A.  G.  Hogbom  on  Gellivare,  p.  20, 
1910.) 

Slickensides,  polished  surfaces  along  faults,  or  fractures  produced  by 
the  rubbing  of  the  walls  upon  each  other  during  movement. 

Soapstone,  metamorphic  rocks,  consisting  chiefly  of  talc.     See  p.  153. 

Soda-granite,  granites  especially  rich  in  soda,  or  whose  soda  exceeds 
the  potash.  Compare  analyses,  p.  33.  See  natron-granite. 

Sodalite-syenites,  syenites  rich  in  sodalite;  close  relatives  of  nephe- 
lite  syenites.  See  Anal.  5,  p.  44.  Sodalite-trachytes  also  occur. 


GLOSSARY.  255 

Soda-syenite,  or  sodium  syenite,  a  syenite  whose  chief  feldspar  is 
albite,  and  in  which  thus  the  soda  exceeds  the  potash.  The  most 
familiar  illustration  is  the  ore-bearing  rock  of  the  Treadwell  mine, 
Alaska.  The  rock  is  chiefly  albite.  If  now  we  define  a  syenite  as  an 
igneous  rock  with  an  alkali-feldspar,  then  it  is  a  syenite.  If,  however, 
we  define  a  diorite,  as  an  igneous  rock  with  plagioclase,  it  would  be 
called  a  diorite  or  albite-diorite.  (See  G.  F.  Becker,  i8th  Ann.  Rep. 
U.  S.  Geol.  Surv.,  3:  I,  1898.) 

Soda-trachytes,  trachytes  rich  in  soda  and  therefore  containing, 
albite,  sodalite,  soda-pyroxenes,  or  soda-amphiboles,  and  marking 
transitions  to  the  phonolites. 

Soggendalite.  a  name  proposed  by  C.  F.  Kolderup  for  a  variety  of 
diabase  that  is  especially  rich  in  pyroxene,  and  that  is  intermediate  be- 
tween true  diabases  and  pyroxenites.  The  type  rock  forms  a  dike  near 
Soggendal,  Norway.  Bergens  Museums  Aarbog,  1896,  159. 

Soil,  surface  earth  mixed  with  the  results  of  the  decay  of  vegetable  or 
animal  matter,  so  as  usually  to  have  a  dark  color. 

SSlvsbergite,  Brogger's  name  for  quartzless  or  quartz-poor  grorudites; 
that  is,  medium  to  finely  crystalline,  dike  rocks,  with  prevailing  alkali- 
feldspar  (mostly  albite  and  microcline)  with  aegirite,  or  in  the  basic 
varieties  with  hornblende  (kataforite),  sometimes  also  with  a  peculiar 
mica.  In  the  most  basic  members  quartz  entirely  fails  and  nephelite 
appears,  (Die  Eruptivgesteine  des  Kristianiagebietes,  I.,  67.) 

Sommaite,  a  name  derived  from  Monte  Somma  (Vesuvius)  and  sug- 
gested by  A.  Lacroix  for  blocks  having  the  composition  of  a  leucite-oli- 
vine-monzonite.  (Cited  by  H.  Rosenbusch,  Mikros.  Phys.,  4th  ed., 
II.,  169.) 

Sondalite,  a  name  proposed  by  Stache  and  von  John  for  a  meta- 
morphic  rock  consisting  of  cordierite,  quartz,  garnet,  tourmaline  and 
cyanite.  Jahrb.  d.  k.  k.  g.  Reichsanst.,  1877,  194. 

Sordawalite,  an  old  name  for  the  glassy  salbands  of  small  diabase 
dikes.  The  sordawalite  was  regarded  as  a  mineral.  It  is  derived  from 
Sordawalar,  a  locality  in  Finland.  Compare  wichtisite. 

Sparagmite,  a  collective  term  used  in  Sweden  for  the  fragmental 
rocks,  especially  feldspathic  sandstones  of  the  Swedish  Jotnian  or  late 
Precambrian  strata.  (Geol.  Guide.  Eleventh  International  Geol. 
Congress,  Pamphlet  2,  p.  8,  1910.)  Sparagmite  means  a  fragmental 
rock,  being  derived  from  the  Greek  for  fragment. 

Spessartite,  a  name  proposed  by  Rosenbusch,  for  those  dike  rocks, 
which,  whether  porphyritic  or  granitoid  in  texture,  consist  of  prevailing 
plagioclase,  hornblende  and  diopside.  Orthoclase  and  olivine  oc- 


256  A  HAND  BOOK  OF  ROCKS. 

casionally  appear.  Massige  Gesteine,  532,  1896.  The  name  is  de- 
rived from  Spessart,  a  group  of  mountains  in  the  extreme  northwest  of 
Bavaria,  but  as  it  has  already  been  used  for  a  variety  of  garnet,  it  is  a 
very  unfortunate  selection. 

Sphenolith,  a  name  suggested  by  C.  Burckhardt  for  a  peculiar 
intrusive  mass  of  dacite  in  the  ridge  called  Las  Parroquias  near  Mazapil, 
Zacatecas,  Mexico.  The  mass  lies  between  steeply  dipping  Jurassic 
and  Cretaceous  beds  and  parallel  with  them  but  its  upper  portion  abuts 
sharply  against  the  overlying  beds  or  cuts  them  at  a  right  angle.  It 
thus  forms  a  sort  of  wedge-shaped  mass  with  a  flat  top  and  received  its 
name  from  the  Greek  for  wedge.  (Geological  Guide  of  the  Tenth 
International  Geol.  Congress,  Pamphlet  26,  p.  33,  1906.) 

Spheroidal,  a  descriptive  term  applied  to  igneous  rocks  that  break  up 
on  cooling  into  spheroidal  masses  analogous  to  basaltic  columns;  also 
used  as  a  synonym  of  orbicular  as  applied  to  certain  granites. 

Spherulites,  rounded  aggregates  or  rosettes,  large  or  small,  of  acicular 
crystals  that  radiate  from  a  center.  They  are  chiefly  met  in  the  micro- 
scopic study  of  acidic,  volcanic  rocks  and  commonly  consist  of  feldspars 
and  quartz.  When  of  one  mineral  they  are  called  by  Rosenbusch  sphere- 
crystals.  They  may  reach  large  size,  though  mostly  microscopic.  See 
p.  26. 

Spilite,  an  early  French  name  for  dense,  amygdaloidal  varieties  of 
diabase. 

Spilosite,  a  spotted,  contact  rock  produced  from  shales  and  slates  by 
intrusions  of  diabase.  It  corresponds  to  the  hornfels  of  granite  con- 
tacts. Zincken  in  Karsten  und  v.  Dechen's  Archiv,  1854,  584. 

Stagmalite,  a  name,  suggested  by  O.  C.  Farrington,  for  stalactites, 
stalagmites,  and  crustiform,  cave  deposits  of  calcite  in  general.  (Field 
Columbian  Museum  Publications  53: 261,  1901.  Geol.  Series  I.,  No.  8.) 

Stalactite,  depending,  columnar  deposits,  generally  of  calcite,  formed 
on  the  roof  of  a  cavity  by  the  drip  of  mineral  solutions.  Compare 
stalagmite. 

Stalagmite,  uprising,  columnar  deposits,  generally  of  calcite,  formed 
on  the  floor  of  a  cavity  by  the  drip  of  mineral  solutions  from  the  roof. 
Compare  stalactite. 

Steatite,  soapstone,  talc  rocks. 

Stock,  a  rudely  cylindrical,  relatively  large,  intrusive  mass.  Good 
illustrations  will  be  found  in  the  Telluride  folio  of  the  U.  S.  Geol.  Surv. 
The  name  is  an  adaptation  of  the  German  word  for  floor  or  story,  and 
originated  in  the  Saxon  tin  mines,  in  which  such  igneous  masses,  im- 
pregnated with  cassiterite  were  mined  in  horizontal  slices,  like  floors. 


GLOSSARY.  257 

Structure,  used  generally  in  America  for  the  larger  physical  features 
of  rocks,  as  against  texture,  which  is  applied  to  the  smaller  ones.  See 
p.  1 6.  Many,  however,  employ  them  interchangeably.  Compare  also 
petrical  and  lithical. 

Stubachite,  a  name  suggested  by  E.  Weinschenk  for  a  more  or  less 
serpentinized  variety  of  dunite,  having  also  diallage,  tremolite,  talc, 
magnetite,  pyrite  and  breunnerite.  (Cited  by  H.  Rosenbusch,  Mikros. 
Phys.,  4th  ed.,  II.,  476.) 

Stylolite,  small,  columnar  developments  in  limestones  or  other  cal- 
careous rocks  that  run  across  the  stratification.  They  appear  to  have 
been  caused  by  some  unequal  distribution  of  pressure  in  consolidation, 
or  by  a  capping  fossil,  as  against  the  surrounding  rock. 

Subhedral,  components  of  igneous  rocks,  which  have  their  own  crystal 
boundaries  in  part  and  are  therefore  intermediate  between  euhedral  and 
anhedral.  Hornblendes  with  prismatic  crystal  faces,  but  without 
terminal  planes,  are  subhedral. 

Subsoil,  the  layer  of  more  or  less  decomposed  and  loose  fragments 
of  country  rock  that  lies  between  the  soil  and  the  bed  rock  in  regions 
not  covered  by  transported  soils. 

Suldenite,  a  name  given  by  Stache  and  von  John  to  gray,  acidic, 
andesitic  porphyrites  in  the  Eastern  Alps.  They  range  from  54-62 
SiO2  and  have,  in  the  prevailing  gray  groundmass,  phenocrysts  of  horn- 
blende, plagioclase,  a  little  orthoclase  and  accessory  augite,  biotite  and 
quartz.  Compare  ortlerite. 

Surficial,  a  general  name,  lately  introduced  by  the  U.  S.  Geological 
Survey,  for  the  untransported  surface,  alteration  products  of  igneous 
rocks. 

Sussezite,  a  special  name  suggested  by  Brogger  for  the  eleolite  por- 
phyry, originally  described  by  Kemp,  from  Beemerville,  Sussex  Co., 
N.  J.  Die  Eruptivgesteine  des  Kristianiagebietes,  1895.  The  name 
was,  however,  applied  years  ago  to  a  hydrated  borate  of  manganese 
and  magnesia,  from  Franklin  Furnace,  N.  J. 

Syenite,  granitoid  rocks  consisting  in  typical  instances  of  orthoclase 
and  hornblende.  In  mica-syenites,  biotite  replaces  hornblende.  In 
augite-syenites,  augite  does  the  same.  For  etymology  and  history  see 
p.  44.  Compare  also  laurvikite,  monzonite,  nordmarkite,  pulaskite, 
sanidinite,  shonkinite,  yogoite. 

Syngenetic,  see  Epigenetic. 

Syssiderite,  Daubree's  name  for  those  meteorites  which  consist  of 
silicates  cemented  together  by  metallic  iron. 


258  A  HAND  BOOK  OF  ROCKS. 

T 

Tachylyte,  Breithaupt's  name  for  a  basaltic  glass.  It  was  originally 
regarded  as  a  mineral  and  was  named  from  two  Greek  words  suggested 
by  its  quick  and  easy  fusibility.  See  analyses  15,  p.  25,  and  descrip- 
tion, p.  26.  Kastner's  Archiv  fiir  die  gesammte  Naturlehre,  VII.,  112, 
1826.  Compare  hyalomelane. 

Taconyte,  a  name  proposed  by  H.  V.  Win  hell  for  the  cherty  or  jas- 
pery,  but  at  times  calcareous  or  more  or  less  quartzitic  rock,  that  encloses 
the  soft  hematites  of  the  Mesabi  Range,  Minn.  Taconytes  are  regarded 
as  in  large  part  altered  greensands  by  J.  E.  Spurr.  The  term  is  current 
in  the  Mesabi  iron  range.  XX.  Ann.  Rep.  Minn.  Geol.  Survey,  124. 
The  name  is  derived  from  Taconic,  E.  Emmons'  rejected  geological 
system. 

Taimyrite,  an  acidic  trachyte,  rich  in  soda,  and  regarded  as  the 
effusive  equivalent  of  nordmarkite.  Chrustschoff ;  Melanges  geol,  et 
paleontol.  Acad.  Sci.  St.  Petersburg,  I.,  153,  1892.  The  original 
locality  is  in  Siberia. 

Talc-schist,  schistose  rocks  consisting  chiefly  of  talc  and  quartz. 
See  p.  142.  Talc  is  also  prefixed  to  several  other  rock  names. 

Taspinite,  a  granitic  rock  enclosing  an  intrusive  mass  of  more  or  less 
metamorphosed  granite-porphyry  in  the  Rofna  valley  of  the  Upper 
Rhine  in  Switzerland.  (Cited  by  H.  Rosenbusch,  Mikros.  Phys.,  4th 
ed.,  II.,  517.) 

Taurite,  a  name  given  by  A.  Lagorio,  to  a  variety  of  rhyolite,  with 
granophyric  or  spherulitic  texture,  rich  in  soda,  and  containing  segirite. 
Guide  to  the  Excursions,  7th  International  Geol.  Congress,  XXXIII., 
27,  1897,  St.  Petersburg. 

Tawite,  a  name  given  by  W.  Ramsey  to  a  very  peculiar  rock  of  both 
granitoid  and  porphyritic  texture  and  consisting  of  pyroxene  and 
sodalite.  It  occurs  in  the  nephelite-syenite  area  of  Kola  in  Finland 
and  is  derived  from  Tawajok,  a  local  geographical  term.  Fennia,  XI., 
2,  1894. 

Taxite,  Loewinson-Lessing's  name  for  lavas,  that,  on  crystallizing, 
have  broken  up  into  contrasted  aggregates  of  minerals  so  as  to  present  an 
apparent  clastic  texture — either  banded,  *'.  e.,  eutaxitic,  or  brecciated, 
i.  e.,  ataxitic.  Bull.  Soc.  Belg.  Geol.,  V.,  104,  1891. 

Tephrite,  basaltic  rocks  containing  lime-soda  feldspar,  nephelite, 
augite  and  basis.  Leucite-tephrites  have  leucite  in  place  of  nephelite, 
and  some  tephrites  have  both.  Tephrites  differ  from  basanites  in  lack- 
ing olivine.  The  name  is  from  the  Greek  for  "  ashen,"  alluding  to  the 
color.  It  is  an  adaptation  of  an  old  form,  tephrine.  Neues  Jahrb., 
1865,  663. 


GLOSSARY.  259 

Teschenite,  a  name  given  in  1861  by  Hohenegger  to  a  group  of 
intrusive  rocks  in  the  Cretaceous  strata  near  Teschen,  Austrian  Silesia. 
They  have,  however,  been  since  shown  to  embrace  such  a  variety  of 
types,  that  the  name  has  little  value,  but  as  analcite  occurs  quite  con- 
stantly in  most  of  them,  many  still  use  the  term  for  diabasic  rocks  with 
this  mineral. 

Texture,   see  structure  and  also  p.  16. 

Theralite,  granitoid  rocks,  consisting  essentially  of  plagioclase,  nephe- 
lite  and  augite,  with  the  common  accessories.  They  were  first  dis- 
covered by  J.  E.  Wolff  in  the  Crazy  Mountains,  Montana.  They  were 
previously  and  prophetically  named  by  Rosenbusch  from  the  Greek  to 
seek  eagerly,  because  this  mineralogical  and  textural  aggregate  was 
believed  to  exist  before  it  was  actually  discovered.  A  spelling  therolite 
is  also  advocated. 

Tholeiite,  Rosenbusch's  name  for  augite-porphyrites,  which,  aside 
from  the  usual  phenocrysts,  have  a  groundmass,  with  but  one  generation 
of  crystals  and  with  a  little  glassy  basis  between  them,  affording  a  tex- 
ture called  intersertal.  Massige  Gest.,  504,  1887. 

Tilaite,  a  name  derived  from  a  locality  in  the  northern  Urals  and 
suggested  by  L.  Duparc  and  F.  Pearce  for  a  variety  of  olivine  gabbro, 
exceptionally  rich  in  diopside.  (Cited  by  H.  Rosenbusch,  Mikros. 
Phys.,  4th  ed.,  II.,  353.) 

Till,  unsorted  glacial  deposits,  consisting  of  boulders,  clay  and  sand. 

Timazite,  a  name  given  by  Breithaupt  to  certain  porphyritic  rocks  in 
the  Timok  Valley  of  Servia,  that  have  since  proved  to  be  varieties  of 
andesite  and  dacite.  Berg,  und  Httttrn,  Zeit.,  1861,  51. 

Tinguaite,  a  name  given  by  Rosenbusch  to  rocks  consisting  of  alkali 
feldspar,  nephelite  and  abundant  aegirite,  which  form  dikes  in  or  near 
areas  of  nephelite-syenite.  It  was  first  applied  to  specimens  from  the 
vicinity  of  Rio  Janeiro,  where  in  the  Serra  de  Tingua  the  rocks  were 
first  discovered  and  described  by  O.  A.  Derby  as  phonolites.  They 
have  since  proved  of  very  wide  distribution  and  not  always  to  accom- 
pany nephelite-syenites  (Black  Hills,  S.  D.).  By  some  the  name  tin- 
guaite  is  regarded  as  an  unnecessary  and  undesirable  synonym  of  pho- 
nolite.  It  first  appears  in  Hunter  and  Rosenbusch,  Tschermarks  Min. 
and  Petrog.  Mitth.,  XL,  447,  1890. 

Tjosite,  a  name  suggested  by  W.  C.  Brogger  for  a  dike-rock,  consist- 
ing of  prevailing  pyroxene,  abundant  apatite  and  magnetite,  with  grains 
of  olivine,  all  set  in  a  paste  of  anorthoclase  rods.  (H.  Rosenbusch, 
Mikros.  Phys.,  4th  ed.,  II.,  705.) 

Toadstone,  an  old  English  name  for  certain,  intruded  sheets  of  amyg- 


260  A  HAND  BOOK  OF  ROCKS. 

daloidafr  basaltic  rocks  in  the  lead  district  of  Cumberland,  England. 
Also  locally  applied  near  Boston  to  a  mottled  felsite,  apparently  spheru- 
litic. 

Toellite,  a  biotite-hornblende-porphyrite,  with  garnets,  that  forms 
dikes  in  mica-schist  and  gneiss  near  Meran,  in  the  Tyrol.  Pichler, 
Neues  Jahrb.,  1873,  940. 

Toensbergite,  a  name  given  by  W.  C.  Brogger  to  certain  very  feld- 
spathic,  syenitic  rocks,  from  Tonsberg,  Norway,  which  are  close  rela- 
tives of  the  anorthosites.  They  differ  from  the  anorthosites  in  their 
smaller  percentages  of  lime  and  higher  percentages  of  alkalies.  Erup- 
tivgest.  d.  Kristianiageb.,  III.,  328,  1899. 

Tonalite,  a  quartz-mica-hornblende  diorite  from  near  Meran  in  the 
Tyrol.  It  was  named  by  vom  Rath  from  Tonale,  a  place  on  Mt.  Ada- 
mello.  Zeit.  d.  d.  g.  Gesellsch.,  XVI.,  249,  1864.  Compare  adamellite. 

Topazfels,  a  brecciated,  contact  rock,  near  granite  contacts  and, 
formed  of  topaz,  tourmaline,  quartz  and  some  rarer  accessory  minerals. 

Tordrillite,  a  name  based  on  the  Tordrilla  Mountains,  Alaska,  and 
suggested  by  J.  E.  Spurr  for  porphyritic  varieties  of  alaskite,  which 
have  a  finely  crystalline  or  aphanitic  groundmass.  See  Alaskite. 

Toscanite,  a  name  proposed  by  H.  S.  Washington  for  a  group  of 
acid,  effusive  rocks  in  Tuscany  (Italian,  Toscana)  and  elsewhere,  which 
are  characterized  mineralogically  by  the  presence  of  basic  plagioclase, 
as  well  as  orthoclase,  and  by  occasional  quartz;  and  chemically  by  high 
silica  and  alkalies,  and  (for  the  acidity)  high  lime,  and  low  alumina. 
They  range  from  63-73  silica  and  are  intermediate  between  rhyolites 
and  dacites.  Journal  of  Geology,  V.,  37,  1897.  Compare  dellenite, 

Touchstone,  see  basanite. 

Tourmaline-granite,  a  variety  of  granite  with  tourmaline  as  the  dark 
silicate.  It  is  usually  due  to  fumarole  action,  and  is  developed  on  the 
borders  of  intrusions  of  normal  granites. 

Trachorheite,  a  name  proposed  by  F.  M.  Endlich  as  a  collective 
designation  for  the  four  rocks,  propylite,  andesite,  trachyte  and  rhyo- 
lite,  as  used  by  von  Richthofen.  Hayden's  reports,  1873,  p.  319. 

Trachy-andesite,  effusive  rocks,  intermediate  between  trachytes  and 
andesites.  Used  by  H.  S.  Washington  for  trachytes  which  have  also 
much  acidic  plagioclase  (andesine  to  oligoclase).  Jour.  Geol.,  V.,  351. 

Trachy-dolerite,  a  name  suggested  by  Abich  for  a  group  of  rocks 
intermediate  between  the  trachytes  and  basalts.  Natur  u.  Zusam- 
mensetzung  der  vulkanischen  Bildungen,  101,  1841.  Compare  Latite. 
Trachy-dolerite  as  used  by  H.  S.  Washington  means  a  trachyte  with 
considerable  basic  plagioclase  (labradorite  to  anorthite).  Jour.  Geology, 
V.,  351- 


GLOSSARY.  261 

Trachyte,  igneous  rocks  of  porphyritic  or  felsitic  texture  consisting 
essentially  of  orthoclase  and  biotite  or  hornblende  or  augite,  one  or 
more.  See  p.  40.  It  was  formerly  used  for  both  rhyolites  and  trachytes 
proper,  or  practically  as  a  field  name  for  light-colored  lavas  and  por- 
phyries. As  such  in  older  reports  it  is  to  be  understood.  Compare 
also  acmite-trachytes  and  pantellerites. 

Trachytic  texture,  a  special  microscopic  name  for  those  ground- 
masses  that  are  made  up  of  rods  of  feldspar,  usually  in  flow-lines,  but 
without  basis. 

Trap,  a  useful  field  name  for  any  dark,  finely  crystalline,  igneous 
rock.  It  is  a  Swedish  name  from  the  occurrence  of  such  rocks  in  sheets 
that  resemble  steps,  ''trappar."  See  p.  90. 

Trass,  a  trachytic  tuff  from  the  Laacher  See,  used  along  the  Rhine 
for  hydraulic  cement. 

Travertine,  calcareous  tufa.  The  name  was  given  by  Naumann  and 
is  of  Italian  origin. 

Trichite,  a  microscopic  term  for  hair-like  crystallites;  so  named  from 
the  Greek  for  hair. 

Tripoli,  a  name  applied  to  diatomaceous  earth  and  to  pulverulent 
silica  derived  by  the  breaking  down  of  cherts  from  some  change  not 
well  understood.  See  p.  in. 

Troctolite,  Bonney's  name  for  a  variety  of  gabbro  consisting  of 
plagioclase  and  olivine  with  very  subordinate  diallage.  The  olivine  may 
be  serpentinized.  Geol.  Magazine,  1885,  439.  Compare  Ossipyte. 

Trowlesworthite,  a  variety  of  granite  which  has  been  so  altered  by 
fumarole  action  that  it  consists  of  fluorite,  orthoclase,  tourmaline  and 
some  quartz,  the  last  named  having  been  largely  replaced  by  the  first. 
The  name  is  derived  from  an  English  locality,  and  was  given  by  Worth, 
Trans.  Roy.  Geol.  Soc.  of  Cornwall,  1884,  180.  Mineralog.  Mag., 
1884,  48. 

Tufa,  the  cellular  deposits  of  mineral  springs,  usually  calcareous  or 
siliceous.  See  p.  108.  Not  to  be  confounded  with  tuff. 

Tuff,  the  finer,  fragmental  ejectments  from  the  explosive  eruptions  of 
volcanoes.  They  may  afterwards  be  water-sorted  or  cemented  to  firm 
rock.  Coarser  ones  are  called  volcanic  breccias,  but  in  neither  do  we 
see  much  sorting  unless  produced  by  subsequent  erosion.  Tufa  is  also 
used  in  this  sense,  but  the  custom  should  be  discouraged. 

Typhonic  rocks,  Brongniart's  name  for  rocks  that  have  come  from  the 
depths  of  the  earth,  i.  e.,  plutonic  and  eruptive  rocks.  Typhon  is  used 
as  a  synonym  of  boss  or  stock. 


262  A  HAND  BOOK  OF  ROCKS. 


Umptekite,  a  name  proposed  by  Ramsay  for  the  border  facies  of  the 
nephelite-syenite  mass  at  Umptek,  Finland.  It  lacks  nephelite  almost 
entirely,  and  contains  perthitic  intergrowths  of  the  alkali-feldspars. 
Arfvedsonite  is  the  chief,  dark  silicate,  but  aegirite  is  also  present.  The 
accessory  minerals  are  numerous.  Fennia,  XL,  2,  1894. 

Unakite,  a  peculiar  granite  consisting  essentially  of  epidote,  pink 
feldspar  and  quartz.  The  name  is  derived  from  the  Unaka  range  of 
mountains  along  the  border  of  North  Carolina  and  Tennessee,  and  was 
first  given  by  F.  H.  Bradley  in  1874.  (Amer.  Jour.  Sci.,  May,  1874, 
519.)  Other  localities  have  since  been  noted.  (See  T.  L.  Watson, 
Idem,  Sept.,  1906,  248.) 

Uralite,  a  special  name  for  that  variety  of  hornblende,  that  is  derived 
by  paramorphism  from  augite.  The  word  is  often  used  as  a  prefix  be- 
fore the  names  of  those  rocks  that  contain  the  mineral.  It  has  also 
suggested  various  rock  names,  such  as  proterobase,  scyelite,  etc.  The 
name  is  derived  from  the  original  occurrence  in  the  Urals.  (G.  Rose, 
Reise  nach  dem  Ural,  II.,  1842,  371.) 

Urtite,  a  name  given  by  W.  Ramsay  to  a  light  colored  rock  of  medium 
grain,  consisting  of  nephelite  in  largest  part,  with  which  is  consider- 
able aegirite  and  a  little  apatite.  When  recast  an  analysis  gave  ne- 
phelite, 82;  aegirite,  16;  apatite,  2.  The  name  is  derived  from  the 
second  part  of  Lujavr-Urt,  the  name  of  the  mountain  where  it  occurs  in 
northern  Finland.  Geol.  Foren.  Forh.,  XVIII.,  463,  1896. 

V 

Valbellite,  a  name  derived  from  the  Valbella  valley  of  Piedmont, 
and  applied  by  R.  W.  Schafer  to  a  dike  of  amphibole-peridotite,  con- 
sisting of  olivine,  brown  hornblende,  bronzite,  pyrrhotite,  spinel  and 
magnetite.  (Cited  by  H.  Rosenbusch,  Mikros.  Phys.,  4th  ed.,  II.,  462.) 

Variolite,  a  special  name  for  a  curious,  border  development  of  dia- 
base intrusions,  which  is  a  very  dense,  finely  crystalline  mass  of  rounded 
spheroids,  largely  spherulitic  in  texture.  They  give  the  rock  a  pock- 
marked aspect  and  hence  the  name,  which  is  a  very  old  one.  Pearl 
diabase  is  synonymous. 

Vaugn6rite,  a  name  derived  from  Vaugneray  near  Lyons,  and  applied 
by  Fournet  in  1836  to  a  dike-rock,  which  is  now  shown  by  Michel-Levy 
and  Lacroix  to  be  an  amphibole-granite.  They  advise  dropping  the 
special  name.  (Bull.  Soc.  mineral  de  France,  1887,  X.,  27.) 

Vein,  strictly  speaking,  the  mineral  matter  which  has  been  deposited 
in  fissures  in  rocks  from  solution.  Water,  assisted  by  dissolved  sub- 


GLOSSARY.  263 

stances,  is  the  almost  invariable  solvent.  The  name  was  doubtless 
originally  given  because  of  the  similarity  of  the  ramifying  and  often 
intersecting  deposits,  to  the  veinsof  man  and  animals.  Vein  is  contrasted 
with  dike,  which  chills  in  a  fissure  from  a  fused  condition.  While  the 
above  simple  statement  holds  true  for  the  vast  majority  of  cases  there 
has  yet  arisen  a  need  of  expansion.  We  find  associated  with  many 
veins,  impregnation  and  oftentimes  replacement  of  the  walls,  such  that 
the  original  supply  fissure  is  a  relatively  small  part  of  the  resulting 
deposit.  We  find  again,  veins,  such  as  the  apatite  deposits  of  Norway, 
obviously  formed  in  large  part  at  least  by  vapors  and  gases  emitted  in 
the  cooling  stages  of  igneous  rocks,  in  the  processes  called  pneumatolitic. 
Volatilization  may  largely  operate  instead  of  mere  solution;  and  re- 
placement of  the  wall  rocks  may  be  extensive.  Again  in  the  formation 
of  pegmatites  we  believe  that  the  process  is  intermediate  between  solu- 
tion at  exalted  temperatures  and  pressures;  and  fusion  in  the  presence 
of  abundant  water  vapor  or  dissociated  hydrogen  and  oxygen,  and 
other  mineralizers.  Some  geologists  describe  pegmatites  as  dikes; 
some  as  veins.  The  point  may  also  be  made  that  since  all  igneous 
magmas  are  regarded  as  solutions,  whose  dissolved  matters  crystallize 
in  the  inverse  order  of  their  solubilities,  no  distinction  can  be  drawn  be- 
tween veins  and  dikes.  But  at  least  in  dikes  almost  the  entire  solvent 
solidifies  with  the  dissolved  matter,  whereas  in  typical  Veins  the  solvent, 
water,  passes  on  after  precipitating  its  burden. 

Ore-deposits  occur  so  often  in  veins,  as  strictly  defined  above,  that 
the  prospector  and  miner  have  applied  the  name  vein  to  any  and  every 
form  of  ore-body.  The  coal-miner  even  speaks  of  coal-seams  as  veins, 
although  this  usage  is  not  to  be  commended  in  carefully  chosen  scientific 
language. 

Venanzite,  a  name  proposed  by  Sabatini,  an  Italian  petrographer, 
for  an  effusive  rock  from  a  small  volcanic  cone  at  San  Venanzo,  Um- 
bria,  Italy.  Venanzite  contains  phenocrysts  of  olivine  in  a  groundmass 
of  melilite,  leucite  and  black  mica,  together  with  a  little  pyroxene, 
nephelite  and  magnetite.  Bolletino  Reale  Comitato  Geologico,  Sept., 
1898.  Rosenbusch  subsequently  described  the  same  rock  under  the 
name  Euktolite,  but  Venanzite  has  priority.  Sitzungsber.  k.  pr.  Akad. 
Wissensch.  Berlin,  VII.,  no,  1899;  Amer.  Jour.  Sci.,  May,  1899,  399. 

Verite,  a  name  derived  from  the  Spanish  locality  Vera,  near  Cabo  de 
Gata,  and  given  by  Osann  to  a  post-Pliocene  glassy  rock,  with  pheno- 
crysts of  biotite  and  microscopic  crystals  of  olivine  and  augite  and  some- 
times plagioclase,  all  of  which  seldom  form  half  the  mass  of  the  rock. 
It  is  a  glassy  variety  of  the  mica-andesites  with  exceptional  olivine.  Z. 
d.  d.  g.  G.,  XLI.,  311,  1889.  Compare  Fortunite. 


264  A  HAND  BOOK  OF  ROCKS. 

Vintlite,  a  quartz-porphyrite  occurring  in  dikes  near  Unter-Vintl,  in 
the  Tyrol.  Compare  toellite  from  the  same  region.  Pichler,  Neues 
Jahrb.,  1871,  262. 

Viridite,  a  microscopic  name  suggested  by  Vogelsang  and  formerly 
used  for  the  small,  green,  chlorite  scales  often  met  in  thin  sections. 
As  their  true  nature  has  now  been  determined,  they  are  generally  called 
chlorite. 

Vitro,  a  prefix  meaning  glassy  and  used  before  many  rock  names,  as 
vitrophyre,  in  order  to  indicate  a  glassy  textur  . 

Vitrophyre,  Vogelsang's  name  for  quartz-porphyries  and  porphyries 
with  glassy  groundmass. 

Vogesite,  Rosenbusch's  name  for  syenitic  dikes,  in  which  the  dark 
hornblendes  or  augites  are  in  excess  over  the  light  colored  feldspars. 
Mass.  Gest.,  1887,  319.  The  name  is  derived  from  Vogesen,  the  Ger- 
man form  of  Vosges. 

Volcanic,  surface  flows  of  lava  as  distinguished  from  plutonic  rocks, 
see  p.  1 6. 

Volcanite,  a  name  proposed  by  W.  H.  Hobbs,  for  an  anorthoclase- 
augite  lava  with  the  chemical  composition  of  dacite.  Bull.  Geol.  Soc. 
Amen,  V.,  598.  The  name  was  suggested  by  the  original  occurrence 
on  the  island  of  Volcano,  one  of  the  Lipari  group,  where  the  rock  is 
met  as  cellular  bombs. 

Volhynite,  a  p^rphyrite  containing  plagioclase,  hornblende  and  bio- 
tite  phenocrysts  in  a  holocrystalline  groundmass  of  feldspar  and  chlorite. 
The  name  was  given  by  Ossovsky,  and  it  is  based  on  the  original  occur- 
rence in  Volhynia.  See  Chrustschoff,  Bull.  Soc.  Min.  France,  1885, 
441. 

Vulsinite,  a  name  suggested  by  H.  S.  Washington  for  a  group  of  rocks 
intermediate  between  trachytes  and  andesites.  They  contain  much 
labradorite  in  addition  to  the  usual  minerals  of  trachyte.  The  name 
is  derived  from  the  Vulsinii,  an  ancient  Etruscan  tribe  inhabiting  the 
region  where  the  type  specimens  were  obtained.  Journal  of  Geology, 
IV.,  547.  Compare  latite  and  trachydolerite. 

W 

Wacke,  an  old  name  for  the  surficial,  clayey  products  of  the  alteration 
of  basalt.  The  syllables  are  still  current  in  graywacke. 

Wash,  a  miner's  term  in  the  West  for  loose,  surface  deposits  of  sand, 
gravel,  boulders,  etc. 

Websterite,  a  name  proposed  by  G.  H.  Williams  for  the  pyroxenites 
near  Webster,  N.  C.,  that  consist  of  diopside  and  bronzite,  with  the 


.GLOSSARY.  265 

latter  porphyritically  developed.  Amer.  Geol.,  VI.,  35,  1890.  The 
name  websterite  had  been  previously  used  by  A.  Brongniart  in  1822  for 
aluminite.  Hauy's  Mineralogie,  II.,  125. 

Wehrlite,  a  name  originally  suggested  by  von  Kobell  for  what  was 
supposed  to  be  a  simple  mineral,  but  which  proved  to  be  a  peridotite 
consisting  of  olivine  and  diallage. 

Weiselbergite,  Rosenbusch's  name  for  those  augite-porphyrites  whose 
groundmass  consists  of  a  second  and  sometimes  third  generation  of 
plagioclase  rods  and  augites,  arranged  in  flow  lines  in  a  glassy  basis. 
Mass.  Gest.,  501,  1887.  Wadsworth  uses  the  name  for  an  altered 
andesite  glass.  Kept,  of  State  Geol.  of  Mich.,  1891-92,  p.  97.. 

Whinstone,  a  Scotch  name  for  basaltic  rocks. 

Wichtisite,  a  glassy  phase  of  diabase,  named  from  a  Finland  locality, 
Wichtis.  Compare  sordavalite. 

Windsorite,  a  name  derived  from  Windsor,  Vt.,  and  applied  by  R. 
A.  Daly  to  a  dike  rock  "  leucocratic,  hypidiomorphic-granular,  com- 
posed essentially  of  alkaline  feldspar  (microperthite  and  orthoclase), 
basic  oligoclase,  quartz  and  biotite,  and  characterized  by  high  alkalies 
(potash  slightly  in  excess  of  soda),  relatively  low  lime  (contained 
essentially  in  the  plagioclase),  low  iron  and  low  magnesia."  (Bull.  209, 
U.  S.  Geol.  Survey,  48,  1903.) 

Wyomingite,  a  name  suggested  by  Whitman  Cross,  for  the  variety  of 
rock  from  the  Leucite  Hills,  Wyoming,  which  consists  almost  entirely 
of  leucite  and  phlogopite.  Small,  acicular  crystals  of  diopside  are  very 
subordinate,  and  apatite  is  also  present.  Amer.  Jour.  Sci.,  Aug.,  1897, 
1 20.  This  is  the  rock  described  by  Zirkel  in  1876  and  was  the  first 
known  occurrence  of  leucite  in  America.  Fortieth  Parallel  Survey, 
VI.,  259- 

X 

Xenogenites,  Posepny's  term  for  mineral  deposits  of  later  origin  than 
the  wall  rock.  The  name  means  foreigners,  and  refers  to  their  later 
introduction.  Compare  idiogenites.  Trans.  Amer.  Inst.  Min.  Eng., 
XXIII.,  205,  1893. 

Xenolith,  a  term  proposed  by  W.  J.  Sollas,  for  included  masses  of 
rock,  caught  up  in  an  igneous  intrusion.  The  term  means  foreign  rock. 
Xenoliths  have  been  subdivided  by  Alfred  Harker  into  accidental, 
or  bodies  foreign  to  the  enclosing  magma;  and  cognate,  or  bodies  con- 
trasted with  the  normal  magma  but  derived  from  it  in  earlier  stages. 
In  illustration  of  the  latter  we  may  cite  glomeroporphyritic  aggregates, 
basic  segregations,  orbicular  or  spheroidal  granites,  and  the  olivine- 
knollen  of  the  basalts.  Cognate  xenoliths  of  single  individual  crystals 


266  A  HAND  BOOK  OF  ROCKS. 

may  be  called  xenocrystals,  such  as  the  occasional  quartz  in  basalt. 
Nat.  Hist.  Ig.  Rocks,  346. 

Xenomorphic,  Rohrbach's  textural  name  for  those  minerals  in  an 
igneous  rock  whose  boundaries  are  determined  by  their  neighbors.  Its 
antithesis  is  automorphic,  which  see.  Xenomorphic  is  synonymous 
with  allotriomorphic,  over  which  it  has  priority.  Tsch.  Mitt.,  1886,  18. 

Yamaskite,  a  name  derived  from  Mt.  Yamaska,  Prov.  Quebec,  and 
applied  by  G.  A.  Young  to  a  very  basic  igneous  rock  of  granitoid  texture 
and  consisting  of  pink,  pleochroic  pyroxene,  basaltic  brown  hornblende, 
anorthite  and  ilmenite.  It  has  less  than  40  per  cent,  silica  and  is 
chemically  near  iacupirangite.  (Ann.  Rep.  Geol.  Surv.  Canada,  17: 
Part  II.,  1907.) 

Y 

Yentnite,  a  name  derived  from  the  Yentna  River,  Alaska,  and  sug- 
gested by  J.  E.  Spurr  for  certain  granitoid  rocks,  consisting  of  oligoclase, 
scapolite  and  biotite,  with  a  few  zircons.  The  scapolite  is  believed  to 
be  an  original  mineral.  Amer.  Jour.  Sci.,  Oct.,  1900,  310. 

Yogoite,  a  name  suggested  by  Weed  and  Pirsson  from  Yogo  peak,  one 
of  the  Little  Belt  Mountains,  Mont.,  for  a  syenitic  rock  composed  of 
orthoclase  and  augite  in  about  equal  amounts.  See  also  sanidinite  and 
shonkinite.  Amer.  Jour.  Sci.,  Dec.,  1895,  473-479. 

Yte,  a  substitute  for  ite,  proposed  by  J.  D.  Dana,  so  that  by  using  it 
in  rock-names  they  would  be  at  once  distinguished  from  mineral  names, 
otherwise,  in  some  instances  the  same. 

z 

Zircon-syenite,  a  name  originally  given  by  Hausmann  to  certain  Nor- 
wegian nephelite-syenites  which  were  rich  in  zircons.  Later  it  was 
practically  used  as  a  synonym  of  nephelite-syenite,  but  is  now  obsolete. 

Zirk elite,  a  name  proposed  by  Wadsworth  in  1887  to  designate 
altered,  basaltic  glasses,  in  distinction  from  their  unaltered  or  tachylitic 
state.  Geol.  Surv.  Minn.  Bull.,  2,  1887,  p.  30. 

Zobtenite,  Roth's  name  for  metamorphic  rocks  with  the  composition 
of  gabbros,  i.  e.,  rocks  not  certainly  igneous.  The  name  is  derived  from 
the  Zobtenberg,  a  Silesian  mountain.  Sitz.  Berl.  Akad.,  1887,  611. 

Zonal-structure,  a  term  especially  used  in  microscopic  work  to  de- 
scribe those  minerals  whose  cross-sections  show  their  successive,  con- 
centric layers  of  growth. 

Zwitter,  a  Saxon  miner's  term  for  a  variety  of  greisen.  Only  of 
significance  in  connection  with  tin  ores. 


INDEX. 


NOTE. — The  index  only  concerns  the  main  portion  of  the  book  and  not  the  Glos- 
sary. Attention  may  be  called  to  the  latter  as  embracing  many  rocks  not  otherwise 
mentioned. 

A. 

Accessory  minerals,  12 
Acmite  in  trachyte,  43 
Acmite-trachyte,  43 
Actinolite,  9 
Actinolitic  slate,  117 
^Egirite,  9 
Alabaster,  115 

Alumina,  molecular  ratios,  170 
Amphibole-andesite,  63 
Amphiboles,  7 
Amphibolites,  139-141 
Analyses  of  rocks,  19 
Andalusite,  126 
Andesine,  6 

Andesite-porphyries,  62 
Andesites,  60 

analyses,  60 

description,  61 

varieties,  62 

synonyms  and  relatives,  62 

alteration  and  metamorphism,  63 

andesitic  tuffs,  63 
Anhydrite,  n 
Anogene,  15 
Anorthite,  5 
Anorthoclase,  5 
Anorthosites,  80 
Anthophyllite,  8 


Anthracite,  112-113 

Apachite,  49 

Apatite,  12 

Aplite,  34 

Apobsidian,  27 

Aporhyolites,  31 

Aqueous  and  Eolian  rocks.  Introduction, 

92 

Arendal,  Norway,  109 
Arfvedsonite,  8,  9 
Argillaceous  limestone,  108 
Arkose,  98 
Atmospheric  weathering,  defined,  121, 122 

rocks  produced  by,  154-157 
Augen  in  gneisses,  132 
Augite,  8,  9 

defined,  9 
Augite-andesite,  63 

-porphyrites,  72 
Augitites,  73 
Aureole,  124 
Auvergne,  trachytes,  43-44 

267 


Bar  theory,  114 
Baryta,  molecular  ratios,  175 
Basalt-porphyries,  71 
Basalts,  69 

alteration  and  metamorphism,  74 

analyses,  69 

basalt  tuffs,  75 

description,  70 

distribution,  75 

synonyms  and  relatives,  72 

varieties,  71 
Basanite,  72 
Batholiths,  15 
Becke,  F.,  cited,  159 
Becker,  G.  F.,  cited,  146,  153,  155 
Beemerville,  N.  J.,  127 

nephelite  syenite,  50 
Biella,  syenite,  analysis,  44 
Binary  granite,  35 
Biotite,  10 
Bituminous  coal,  112-113 

shales,  101 

Blackband  iron  ore,  112,  118 
Black  Hills,  S.  D..  phonolites,  50 

trachyte,  43 
Bosses,  15 
Bostonite,  43 

Brazil,  weathered  rocks  of,  154 
Breccias,  93,  126 

eruptive  breccias,  93 

friction  breccias,  93 

talus  breccias,  93 
Broegger,  W.  C..  cited,  162 
Bronzite,  9 
Bytownite,  6 


Calcareous  sandstones  and  marls,  103 

analysis,  103 

metamorphism  of,  104 

mineralogical  composition,  103 

occurrence,  104 

varieties,  103 
-alcarenites,  107 
Calcareous  shales,  107 
Calcilutites,  108 
Calcirudites,  107 
Calcite,  n 
Calc-schist.  139 
mptonite,  66 
Cancrinite,  53 


268 


INDEX. 


Carbonaceous  sediments,  112 
Carbonates,  rock-forming,  n 
Carbonic  acid,  molecular  ratios,  173 
Cement-rock,  109 
Chalcedony,  109,  no 
Chemical  analysis,  calculations  from,  ig 
recasting,  161 

composition,  diagrams  of,  20 
influence  of,  17 

elements  important  in  rocks,  3 
Chert,  109-111,  116-118. 
Cherty-iron  carbonates,  116-118. 

limestone,  108 
Chiastolite,  126 
Chlorides,  mineral,  12 
Chlorine,  molecular  ratios,  174 
Chlorite,  n 

schist,  141-143 
Christiania,  Norway,  128 

syenites,  46 

Chromic  oxide,  molecular  ratios,  175 
Chromite,  n 
Citric  acid,  119 
Clarke,  F.  W.,  cited,  3 
Classification  scheme,  igneous  rocks,  22, 
23 

of  rocks,  principles  of.  12 
Clay,  analysis,  100 

description,  102 

iron  stone,  112,  118 
Cobalt  oxide,  molecular  ratios,  175 
Conanicut,  R.  I.,  125 

minette,  46 
Conglomerates,  126 
Consanguinity,  91 
Contact  metamorphism,  121-124 

external  effects,  122 

internal  effects,  121 

zones,  122 

Coral  limestone,  107 
Corniferous  limestone,  Ij8 
Cornwall,  Pa.,  129 
Cortlandt  series,  quartz-diorite,  60 

series,  contacts,  128 
Crawford  Notch,  N.  H.,  127 
Crinoidal  limestone,  109 
Cripple  Creek,  Col.,  phonolites,  50 
Cross,  Whitman,  cited,  170 
Crugers,  N.  Y.,  126 
Gratification,  114 
Crystalline  limestone,  148-150 

schists,  137 

Crystallization,  order  of,  20,  21 
Cupric  oxide,  molecular  ratios,  165 
Custer  Co.,  Col.,  syenite.  46 

trachyte,  43 
Cyanite,  n 
Cycle  of  deposition.  95 


Dacite-porphyry,  S7 
Dacites,  56-58 


Dana,  J.  D.,  cited,  139 
Degeneration  of  rocks,  155 
Derby,  O.  A.,  cited,  154 
Diabases,  76 

analyses,  76 

texture  of,  77 
Diallage,  9 

Diatomaceous  earth,  109,  ill 
Dikes,  15 
Diopside,  9 
Diorite-porphyries,  62 
Diorites,  64 

alteration  and  metamorphism,  66 

analyses,  64 

distribution,  66 

mineralogical  composition,  65 

varieties,  66 
Dissolved  vapors,  18 
Ditroite,  52 
Dolerite,  71 
Dolomite,  n,  108 

crystalline,  148-150 
Drachenfcls-trachyte,  44 
Dunkirk,  Md.,  in 
Dykes,  see  dikes 
Dynamic  metamorphism,  132 


Earth,  composition  of  the  crust,  3 
Eclogite,  142,  143 
Effusive  rocks,  16 
Eleolite.  see  nephelite 

syenite,  see  nephelite  syenite 
Enstatite,  9 
Eolian  sands,  95 
Epidote,  n 

schist,  142,  143 
Essential  minerals,  12 
External  contact  metamorphism,  126 
Extrusive  rocks,  16 


Feldspars,  5 

Feldspathoids,  6 

Felsite,  30 

Felsitic  texture,  17 

Ferric  oxide,  molecular  ratios,  170 

Ferromagnesian  silicates,  4 

Ferrous  oxide,  molecular  ratios,  171 

Ferruginous  organic  rocks,  112 

Flexible  sandstone,  144 

Flint,  117 

Fluorine,  molecular  ratios,  174 

Foyaite,  52 

Franklin  Furnace,  N.  J.,  109 

Freiberg  minettes,  46 

gneisses,  136 

Freshwater  limestone,  analyses,  105 
Friction  breccia,  91 
Fusibility  of  magmas,  17 
Fusing  points  of  minerals,  21 
of  rocks,  22 


INDEX. 


269 


Gabbro.  78 

alterations   and   metamorphism,    84 

analyses,  78 

distribution,  84 

mineralogical  composition,  80 

varieties,  80 
Gabbro-porphyries,  71 
Garnet,  n 

Generations  of  minerals,  21 
Geyserite,  no,  in,  116-118 
Gieseckite-porphyry,  49 
Glasses,  25 

analyses,  25 

distribution  of,  27 

geological  occurrence,  27 

relationships,  27 

varieties,  26 
Glassy  texture,  16 
Glauconite,  103 
Glaucophane  schist,  142,  143 
Gneisses,  133-136 
Gordon,  C.  H.,  cited,  133 
Grabau,    A.    W.,    cited    on    limestones. 

107 

Granite-porphyries,  31 
Granites,  33 

analysis  of,  33 

distribution,  38 

metamorphism  of,  37 

mineralogy  of,  34,  35 

occurrence,  36 

relationships,  36 

uses  of,  37 

varieties,  34 
Granitoid  texture,  17 
Grano-diorites;  36 

diorite,  59,  60 
Granulite,  136,  137 
Graphic  granite,  35 
Graphite,  12 
Graphite  schist,  143 
Gravels  and  conglomerates,  96 

metamorphism  of,  96 

occurrence,  96 
Greensands,  104 
Green  schists,  142 
Greisen,  130 
Groundmass,  17 
Grorudite,  32 
Grflnerite,  118 
Gypsum,  n,  114,  115 


Halite,  12 

Harker,  Alfred,  cited,  124 
Hawes.  G.  W.,  cited,  127 
Hematite,  n 

Highwood  Mtns.,  Mont.,  SO 
Hillebrand.  W.  F.,  153 
Hoboken,  N.  J..  127 
Hollick.  A.,  cited,  128 


Hornblende,  8,  9 
Hornblende-schists,  139-141 
Hornblendite,  83 
Hornfels,  126,  127 
Hornstone,  117 
Hyalomelane,  26 
Hydraulic  limestone.  108 
Hydromica-schist,  139 
Hypersthene,  9 
Hypersthene-andesite,  63 
fels,  80 

I 

Iddings.  J.  P.,  cited,  170 
Igneous  rocks,  15 

review  of,  88 

determination  of,  90 

field  observations,  91 

mineralogy  of,  89 

range  of  composition,  88 

typical  textures,  89 

table  of,  23 
Ilmenite,  n 

Infusorial  earth,  109-111 
Internal  contact  metamorphism,  124 
Intratelluric,  18 

Iron  Mtn.,  Mo.,  trachyte-porphyry,  43 
Itacolumite,  144 

K 

Kaolinite.  ir 
anals.,  92 
Kemp,  J.  F.,  cited,  127,  128 
Keratophyre.  43,  58 
Kersanite,  66 
Kimberlite.  83 
Knotty  schists,  127 

slates,  127 
Kulaites,  72 

Labradorite,  6 
Labradorite-rock,  80 
Laccoliths,  15 

Lake  Champlain  trachyte,  43 
Laterite,  154 
Latite,  43 

Laurdalite.  anals.,  51 
Leucite,  6 
Leucite-basalt,  72 
Leucite-basanite,  72 
Leucite  Hills,  Wyo.,  50 
Leucite-phonolitc,    anals.,    Rieden;    Ger- 
many. 46 

defined,  50 
Leucite  rocks,  50 

syenite  anals..  51 

tephrite,  72 
Leucitophyre,  50 
Liebenerite-porphyry,  49 
Lignite,  112,  113 
Limburgites,  73 
Lime,  molecular  ratios,  172 
Limestone,  crystalline,  148-150 

alteration.  150 


270 


INDEX. 


Limestone,  composition,  148 

occurrence,  150 

varieties,  149 
Limestones,  105 

analyses,  105 

at  igneous  contacts,  128 

origin  of,  106 
Limonite,  II 
Liparite,  defined,  31 
Litchfieldite,  anals.,  51 

defined,  52 

Lithia,  molecular  ratios,  175 
Lithographic  limestone,  109 
Lithophysae,  27 
Living  organisms,  analyses  of  calcareous 

parts,  105 

Local  metamorphism,  121 
Loess.  99 
Luxullianite,  35 
Lyell,  Charles,  121 

M 

Magnesia,  molecular  ratios,  161 

Magnet  Cove,  Ark.,  nephelite-syenite,  50 

Magnetite,  n 

Magnesian  limestone,  108 

Malacolite,  9 

Manganous  oxide,  molecular  ratios,  175 

Marble,  see  crystalline  limestone 

Marls,  analyses,  103 

Matthew,  W.  D.,  cited.  128 

Mechanical  sediments,  90 

Melaphyre,  72 

Melilite.  7 

basalt,  74 

Merrill,  G.  P.,  cited,  155 
Metamorphic    rocks,    determination    of, 

157.  158 
Metamorphism,  121 

of  limestones,  109 
Meteorites,  85 
Miarolitic,  17 
Mica-andesite,  63 
Micas,  9 

Mica-schists,  137,  139 
Mica-syenites,  45 
Microcline,  5 
Micro-granites,   36 
Mineralizers,  17,  1 8 
Minerals,  accessory,  12 

essential,  12 

primary,  12 

secondary,  12 
Minette,  45 

Minor  schists,  141-143 
Mixed  zone,  119 
Molecular  proportions,  162 

ratios,  162 

Molecular  volumes,  159 
Molten  magmas,  nature  of,  20 
Monzonites,  45 
Mt.  Willard,  N.  H.,  127 


Muscovite,  10 

Necks,  defined,  15 
Nephelite,  6 

Nephelite-basalt,  anals.,  72 
Nephelite-syenite,  analyses,  51 

description,  51-53 

porphyry,  49 
Nevadite,  31 

Nickel  oxide,  molecular  ratios,  175 
Norite,  80 

Novaculite,  analysis,  97 
defined,  98 

o 

Obsidian,  26 

cliff,  cited,  27 
Ochsenius,  cited,  114 
Oligoclase,  5 
Olivine,   10 

Olivine-free  6asalts,  72 
Onyx  marble,  114 
Ophicalcites,  150-153 

alteration,    153 

composition,  150 

distribution,  153 

varieties,  152 
Ophiolite,  see  ophicalcite 
Oolitic  limestones,  108 
Orbicular  granite,  36 
Orendite,  50 

Organic  remains  not  limestone,  109 
Orthoclase,  5 

Dxides,  common  in  rocks,  n 
Oxygen  quotient,  161,  162 


Paisanite,  32 
Pantellerite,  43,  58 
Paramorphism,  75 
Peat,  112-113 
Pegmatite,  35 
Pele's  hair,  anals.,  14 
Peridotites,  81 
Perlite,  26 

Petrographic  provinces,  91 
Petrosilex,  119 
Petite  Anse,  La.,  115 
Phanerocryst,  17 
Phenocrysts,  17 
Phlogopite,  10 

'honolite-obsidian,  anals.,  25 
?honolite-porphyry,  49 

honolites,  analyses,  46 
description,  47-51 

hosphates,  minerals,  12 

hosphoric  pentoxidc,  molecular  ratios, 
174 

hthanites,  102 

hyllite,  139 

Jilot  Knob,  Mo.,  trachyte-porphyry,  43 
'irsson,  L.  V.,  cited,  127,  163,  170 
Pisolitic  limestone,  108 


INDEX. 


271 


Pitchstone,  26 

Plagioclase  feldspars,  5 

Plauenscher  Grund  syenite,  45 

Plutonic  rocks,  16 

Popes  Mills,  Md.,  in 

Porphyrite,  63 

Porphyritic  texture,  16 

Porphyry,  a  pre-tertiary  trachyte,  43 

Potash  molecular  ratios,  173 

Precipitates  from  solution,  113 

Predazzo,  Tyrol,  128 

Pressure,  influence  of,  18 

Primary  minerals,  12 

Principles  of  rock  classification,  12 

Propylite,  63 

Pumice,  26 

Pyrite,  12 

Pyroxenes,  7 

Pyroxenites,  81 

Pyrrhotite,  12 

Q 

Quartz,  n 

basalt,  73 
Quartz-diorite,  59 

porphyry,  54,  58 
Quartzites,  144-145 
Quartz-keratophyres,  32 

pantellerites,  31 

porphyries,  31 

porphyrite,  54 

trachyte,  31 

R 

Rate  of  cooling,  influence  of,  18 
Regional  metamorphism,  121-122 
Regionally   metamorphosed   rocks,    131- 

133 

Residual  magma,  21 
Rhyolite  granite  series,  28 

porphyries,  31 
Rhyolites,  28 

alteration  of,  32 

analyses  of,  28 

distribution,  32 

general  description,  30 

relationships,  32 

synonyms,    31 
Rhyolite,  alteration,  32 

composition,  28-30 

distribution,  32 

varieties,  31 
Rhyolite  tuffs,  33 
Richmond,  Va.,  Ill 
Rieden,  Germany,  leucite  rocks,  50 
Rock,  definition  of,  I 

magmas.  17 

salt,  12,  114-115 
Rocks,  chemical  composition  of,  3 

physical  range  of,  2 

principles  of  classification,  12 

the  three  great  classes,  of,  13,  14 
Rosenbusch,  H.,  cited,  127 


St.  John,  N.  B.,  128 
Sands  and  sandstones,  97 

analyses,  97 

metamorphism  of,  99 

mineralogical  composition,  98 

occurrence.  99 

varieties,  98 
Sandstones,  126 
Sanidine,  5 
Saprolite,  155 
Scapolite,  II 
Schists,  137 
Scorias,  26 

Secondary  minerals,  12 
Sedimentation,  94 

agents  of,  95 
Selenite,  115 
Serpentines,  150-153 

alteration,  153 

composition,  150 

distribution,  153 

varieties,  152 
Seven  Devils,  Idaho,  129 
Shales,  126 

and  clays,  100 

analyses,  100 

metamorphism  of,  102 

mineralogical  composition,  101 

occurrence,  102 

varieties,  101 
Sheets,  15 
Shonkinite,  45 
Siderite,  n 

Silica,  molecular  ratios,  169 
Silicates,  4 
Siliceous  limestones,  105 

oolite,  116-118 

sinter,  109- in 
Mllimanite,  II 
Slates,  114,  145-148 

alteration.  147 

composition,  145 

development,  145,  146 

distribution,  148 
Slates,  varieties,  145 
Slaty  cleavage,  147 
Smyth,  C.  H.,  Jr.,  cited,  153,  158 
Soda,  molecular  ratios,   172 
Soapstone,  150-153 
Sodalite-syenite,  46 
Sorby,  H.  C.,  cited,  146 
Sparta,  N.  J.,  109 
Specific  gravity,  20 
Spheroidal  granite,  36 
Spherulites,  26 
Stalactites,  114 
Stalagmites,  114 
Standard  minerals,  170 
Stassfurt,  115 
Staurolite,  n 
Steatite  (see  soapstone) 


2/2 


INDEX. 


Stratification,  95 
Strontia,  molecular  ratios,  175 
Structure,  significance  of  term,  16 
Sulphates,  mineral,  n 
Sulphides,  important,  12 
Sulphur,  molecular  ratios,  174 
Sulphuric  anhydride,  molecular  ratios,  1 74 
Surficial,  defined,  155 

rocks,  155 
Sussexite,  49 
Syene,  Egypt,  44 
Syenite-porphyries,  42 
Syenite  recalculated,  163 
Syenites,  analyses,  44 

description,  44,  45 


Tabulation  of  igneous  rocks,  23 
Tachylyte,  anals.,  25 
Talc  in  soapstone,  153 

schist,  142 

Temperature,  influence  of,  17 
Tephrite,  72 

Texture,  meaning  of  term,  16 
Textures,  factors  governing,   17,   18,  24 

of  rocks,  16,  17 
Theralite,  83 
Tinguaite,  49 

Titanic  acid,  molecular  ratios,  174 
Titanite,  10 
Tonalite,  60 
Topaz,  it 
Tourmaline,  it 

granite,  35 
Trachyandesite,  43 
Trachydolerite,  43 
Trachyte-porphyries,  42 
Trachyte-syenite  series,  40 


Trachytes,  40 

analyses,  38,  40 

description,  41 

mineralogy  of,  41 

synonyms,  42 

textures,  of  42 
Trachytic  texture,  42 
Travertine,  108 
Tremolite  in  marble,  149 
Tripoli,  in,  117 
Tyndall,  John,  cited,  146 

U 

Ultra-basic  Igneous  rocks,  85 


Van  Hise,  C.  R.,  cited.  158 
Vesuvianite,  129 
Volcanic  rocks,  16 
Volcanite,  43 
Vosges  Mtns.,  127 
Vulsinite,  43 

w 

Wadsworth,  M.  E.,  cited,  157 
Washington,  H.  S.,  cited,  168,  170 
Water,  molecular  ratios,  173 
Waterlime,  109 
Weathering,  121,  154 
Websterite,  analysis,  81 
Weed.  W.  H..  cited,  163 
Westmoreland,  Eng.,  123 
White  Mtns.,  N.  H.,  syenite,  46 
Williams,  G.  H.,  cited,  128 
Wyomingite,  50 

z 

Zeolites,  n 

Zircon,  10 

Zirconia,  molecular  ratios.  174 


I 


1580^495920 

t* 


UC  SOUTHERN  REGIONAL  LIBRARY  FACILITY 


AA    000766381    8 


BRANCH 
'ERS!         F  CALIFORNIA 
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